Kariri
Kariri was a language isolate once spoken in Brazil.
An isolate: no relationship to any other known language has been demonstrated.
The Karirí languages, generally considered dialects of a single language, are a group of languages formerly spoken by the Kiriri people of Brazil. It was spoken until the middle of the 20th century; The 4,000 ethnic Kiriri are now monolingual Portuguese speakers, though a few people know common phrases and names of medicinal plants. A revival of the Dzubukuá variety has been ongoing since 1989.
From Wikipedia · Kariri languages · CC BY-SA 4.0
Most similar languages
Vocabulary compares 40 basic words (ASJP); grammar compares 195 features (Grambank). Similar is not always related.