Born in 718 to Azdi parents of modest means, al-Farahidi was from southern Arabia (modern day Oman) and later moved to Basra, Iraq. Originally an Ibadi (a sect of Islam), there is a weak opinion saying he became Sunni but majority of the people of Arabia have said he died an ibadi. He was one of the companions of jaber bin zayd the founder of ibadism. Growing up he moved to Iraq, where he lived a simple and pious life; he was only a second-generation Muslim, his parents having converted to the faith. It was said that al-Farahidi's father was the first person to be named "Ahmad" after the time of the Muslim prophet Muhammad. His nickname "Farahidi" differed from his tribal name as one of his ancestors was named Furhud; a furhud is a young lion and the plural is farahid. The modern-day descendants of his tribe are the Zahran tribe residing primarily in the Al Bahah Province of Saudi Arabia.
While in Basra, he was a student of Abu 'Amr ibn al-'Ala'. Unlike his contemporaries among Arab and Persian men of letters, al-Farahidi refused both lavish gifts from the ruling class and sinking to the level of maliciously slandering and rumor-mongering against his intellectual rivals. While in Basra, he made a living as a language teacher. He was said to have lived a pious and simple life, performing the pilgrimage to Mecca almost every year. Al-Farahidi lived in a small house made of reeds and once remarked that once he shut his door, his mind did not go beyond it. Although some of his students amassed wealth through their own teaching, most of al-Farahidi's income was from falconry and a garden he inherited from his father.
Al-Farahidi's date of death has been listed as both 786 and 791 CE. His preoccupation with deep thoughts led to his death. It is said that one day, al-Farahidi was attempting to work out an accounting system in his head which would save his maidservant from being cheated out of money by a green grocer; he then absent-mindedly bumped into a pillar of a mosque he had wandered into and sustained a fatal fall.
Al-Farahidi's eschewing of material wealth has been noted by a number of biographers. In his old age, the son of Habib ibn al-Muhallab and reigning governor of the Muhallabids offered al-Farahidi a pension and requested that the latter tutor the former's son. Al-Farahidi declined, stating that he was wealthy though possessing no money, as true poverty lay not in a lack of money, but in the soul. The governor reacted by rescinding the pension, an act to which al-Farahidi responded with the following lines of poetry:
"He, Who formed me with a mouth, engaged to give me nourishment till such a time as He takes me to Himself. Thou hast refused me a trifling sum, but that refusal will not increase thy wealth."
Embarrassed, the governor then responded with an offer to renew the pension and double the rate, which al-Farahidi still greeted with a lukewarm reception. Al-Farahidi's apathy about material wealth was demonstrated in his habit of quoting Akhtal's famous stanza: "If thou wantest treasures, thou wilt find none equal to a virtuous conduct."
Al-Farahidi distinguished himself via his philosophical views as well. He reasoned that a man's intelligence peaked at the age of forty - the age when the Islamic prophet Muhammad began his call - and began to diminuish after sixty, the point at which Muhammad died. He also believed that a person was at their peak intelligence at the clearest part of dawn.
In regard to the field of grammar, al-Farahidi held the realist views common among early Arab linguists yet rare among both later and modern times. Rather than holding the rules of grammar as he and his students described them to be absolute rules, al-Farahidi saw the Arabic language as the natural, instinctual speaking habits of the Bedouin; if the descriptions of scholars such as himself differed from how the Arabs of the desert naturally spoke, then the cause was a lack of knowledge on the scholar's part as the unspoken, unwritten natural speech of pure Arabs was the final determiner. Al-Farahidi was distinguished, however, in his view that the Arabic alphabet included 29 letters rather than 28 and that each letter represented a fundamental characteristic of people or animals. His classification of 29 letters was due to his consideration of the combination of Lamedh and Aleph as a separate third letter from the two individual parts.
Al-Farahidi was already a household name in the Arab world before his death; he was as respected as the possibly mythical figure Abu al-Aswad al-Du'ali in Arabic philology, was the first person to codify the complex metres of Arabic poetry, and has been referred to as the outstanding genius of the Muslim world. Sibawayh and Al-Asma'i were among his students, with the former having been more indebted to al-Farahidi than to any of his other teachers; Sibawayhi quotes al-Farahidi 608 times in his infamous Kitab, more than any other authority. At any point in the Kitab when Sibawayh says "I asked him" or "he said" without mentioning a name, he is referring to al-Farahidi. Both the latter and the former are historically the earliest and most significant figures in respect to the formal recording of the Arabic language.
In addition to the Arabic language, al-Farahidi was also well versed in astronomy, mathematics, Islamic law, music theory and Muslim prophetic tradition. His prowess in the Arabic language was said to be drawn, first and foremost, from his vast knowledge of Muslim prophetic tradition as well as exegesis of the Qur'an. The Al Khalil Bin Ahmed Al Farahidi School of Basic Education in Rustaq, Oman is named after him.
In addition to his work in prosody and lexicography, al-Farahidi was also considered the first person to write about the rules governing the use of metre in Arabic poetry (ʻarūḍ) and musicology in the Arabic language. He is often referred to as a genius by historians and was considered not only a scholar, but an inventor of ideas.
Al-Farahidi was the first of many linguists who would author works on the topic of cryptography and cryptanalysis. He wrote a book on cryptography titled the "Book of Cryptographic Messages." The lost work contains many "firsts", including the use of permutations and combinations to list all possible Arabic words with and without vowels. Later Arab cryptographers explicitly resorted to al-Farahidi's phonological analysis for calculating letter frequency in their own works.
Al-Farahidi is also credited with the current standard for Arabic diacritics; rather than a series of indistinguishable dots, it was al-Farahidi who introduced different shapes for the vowel diacritics in Arabic, which simplified the writing system so much that it hasn't been changed since. He also began using a small letter shin to signify the shadda emphatic mark. Al-Farahidi's style for writing the Arabic alphabet was much less ambiguous than the previous system where dots had to perform various functions, and while he only intended its use for poetry it was eventually used for the Qur'an as well.
Kitab al-Ayn was the first dictionary ever written for the Arabic language. Ayn is the deepest letter in Arabic, al-Ayn also means a water source in the desert. It was titled "the source" because the goal of its author was to clarify those words which were composed the original or source Arabic vocabulary.
Al-Farahidi's first work was in the study of Arabic prosody, a field for which he is credited as the founder. Reportedly, he performed the Hajj pilgrimage to Mecca while a young man and prayed to God that he be inspired with knowledge no one else had. When he returned to Basra shortly thereafter, he overheard the rhythmic beating of a blacksmith on an anvil, he immediately wrote down fifteen metres around the periphery of five circles which were accepted as the basis of the field and still accepted as such in Arabic language prosody today. Three of the meters were not known to Pre-Islamic Arabia, suggesting that al-Farahidi may have invented them himself. He never mandated, however, that all following Arab poets must necessarily follow his rules without question, and even he was said to have knowingly broken the rules at times.