borrowed from French, borrowed from a Tupi word recorded as unaü in the 17th century
The use of the word unau as a generic name for the two-toed sloth (and ai as a name for the three-toed sloth) is owed to Georges-Louis Leclerc de buffon, who so denominated them in Histoire naturelle, générale et particulière, tome treizième (Paris, 1765), p. 34 ff. The word ai is well-documented (see note at ai), but unau is apparently reported only in a description of the sloth by the French Capuchin priest Claude d'Abbeville, who resided on the island of Maranhão (French Maragnan, now called in Portuguese ilha de São Luís or Upaon-Açu) in 1612-15: "Il y en a de deux sortes, aucuns sonts grands enuiron comme les Lieures qu'ils appellent Vnaü, & les autres sont deux fois presque plus grands qu'ils appellent Vnaü Oaüssou & d'autant plus monstrueux" (Histoire de la mission des pères capucins en l'Isle de Maragnan et terres circonvoisines, Paris, 1614, p. 252). ("There are two kinds of them [sloths], some are about as large as hares, which they call unaü, and others are almost two times as large, which they call unaü oaüssou, and all the more monstrous.") Though the suffixed word oaüssou [wasu] means "large" in Tupi, and a number of other names cited by d'Abbeville are familiar from other records, unaü does not appear to be attested elsewhere.