According to the Book of Mormon, the Anti-Nephi-Lehies (/ˈæn.taɪ ˈniː.faɪ ˈliː.haɪz/) were a group of people who, after a significant religious conversion, made a covenant that they would not participate in war, and buried their weapons.
The plural form of the name, Anti-Nephi-Lehies, was taken on by Lamanite Christian converts. Later, they were called Ammonites to honor the primary missionary of their conversion. Anti-Nephi-Lehies differed from other Book of Mormon people in that they were pacifists as a reaction to their previous extreme warlike behavior. Anti-Nephi-Lehies preferred death over killing in self-defense. Anti-Nephi-Lehi, who succeeded his father as king of all the Lamanite lands except the land of Ishmael, was also one of the converted Lamanites, and a brother to Lamoni.
The term "anti" has perplexed readers as the Lamanites made a covenant to serve the Lord and thus align themselves with the Nephites. These readers assume the Latin/Greek meaning of the prefix "anti", which means "opposing" or "against". However, the Book of Mormon record states that it was written in reformed Egyptian, so a Greek or Latin meaning is unlikely. "Anti" may be a reflex of the Egyptian "nty:", he of, the one of. Thus, rather than having the sense "against", it may have the meaning "the one of Nephi and Lehi".
An obvious explanation of "anti" is meaning "not". Their challenge of distinguishing themselves as a body of Lamanite converts without using the term "Laman", hence "The Lehies not of Nephi" or "The not-Nephite Lehies." Another possibility is in Alma 21:11 where a village is named "Ani-Anti", as perhaps the name Anti was derived from this village and it is merely a name.