Nihali, also known as Nahali or erroneously as Kalto, is a threatened language isolate spoken in west-central India (in Madhya Pradesh and Maharashtra) with approximately 2,000 people (in 1991) out of an ethnic population of 5,000. The Nihali tribal area is just south of the Tapti River, around the village of Tembi in Nimar district of Central Provinces during British Raj, now in Madhya Pradesh. Speakers of the Nihali language are also present in several villages of the Buldhana district in Maharashtra such as Jamod, Sonbardi, Kuvardev, Chalthana, Ambavara, Wasali, and Cicari. There are dialectal differences between the Kuvardev-Chalthana and the Jamod-Sonbardi varieties.
The language has a very large number of words adopted from neighboring languages, with 60–70% apparently taken from Korku (25% of vocabulary and much of its morphology), from Dravidian languages, and from Marathi, but much of its core vocabulary cannot be related to these or other languages, such as the numerals and words for "blood" and "egg". Scholars state that less than 25% of the language's original vocabulary is used today. There are no longer any surviving monolingual speakers of the language. Those that are well-versed in modern Nihali are likely to speak varieties of Hindi, Marathi, or Korku as well.
Franciscus Kuiper was the first to suggest that it may be unrelated to any other Indian language, with the non-Korku, non-Dravidian core vocabulary being the remnant of an earlier population in India. However, he did not rule out that it may be a Munda language like Korku. The Endangered Languages Project surmises a relationship with Kusunda, Ainu and the Andamanese languages as part of Joseph Greenberg's Indo-Pacific hypothesis. The Nihali have long lived in a symbiotic but socially inferior relationship with the Korku people, and are bilingual in Korku, with Nihali frequently spoken to prevent the Korku from understanding them. The original Nihali were poor laborers who served as agricultural workers for communities other than their own. Kuiper suggested that the differences might also be argot, such as a thieves' cant. Norman Zide described the situation this way:
Nihali's borrowings are far more massive than in such textbook examples of heavy outside acquisition as Albanian. It seems to compare more in this repect [sic] to some of the more "broken-down" dialects of Gypsy, such as those spoken in the United States and Western Europe. The recent history of the Nihalis includes a massacre organized by one of the rulers in the area in the early nineteenth century, this apparently in response to their increasingly destructive marauding. Since then, the group---decimated in size---has functioned largely as raiders and thieves, with traditional outside associates who disposed of the stolen goods. The group has long been multilingual, and uses Nihali as a more or less secret language which is not ordinarily revealed to outsiders. Earlier investigators attempting to learn the language were, apparently, deliberately rebuffed or misled.
Kuiper's assumptions stem from the fact that several lower-class groups in India have had secret languages. These secret languages were used as a means to conceal communication from oppressive upper class groups. Today, several of the Korku speakers refuse to acknowledge the Nihali language. Korku natives describe the emergence of the Nihali into their community as a civil disturbance.
The Nihali live similarly to the Kalto; this, combined with the fact that Kalto has often been called Nahali, has led to confusion of the two languages in the literature.