The Collaborative International Dictionary of English v.0.48:

Baboon \Bab*oon"\, n. [OE. babewin, baboin, fr. F. babouin, or
   LL. babewynus. Of unknown origin; cf. D. baviaan, G. pavian,
   baboon, F. babine lip of ape, dogs, etc., dial. G. b[aum]ppe
   mouth.] (Zool.)
   One of the Old World Quadrumana, of the genera Cynocephalus
   and Papio; the dog-faced ape. Baboons have dog-like muzzles
   and large canine teeth, cheek pouches, a short tail, and
   naked callosities on the buttocks. They are mostly African.
   See Mandrill, and Chacma, and Drill an ape.
   [1913 Webster]

WordNet (r) 3.0 (2006):

baboon
    n 1: large terrestrial monkeys having doglike muzzles