The Collaborative International Dictionary of English v.0.48:

Tupi \Tu"pi\, pr. n.
   An Indian of the tribe from which the Tupian stock takes its
   name, dwelling, at the advent of the Portuguese, about the
   mouth of the Amazon. Also, their language, which is the basis
   of the Indian trade language of the Amazon.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Tupi
    n 1: a member of the South American Indian people living in
         Brazil and Paraguay
    2: the language spoken by the Tupi of Brazil and Paraguay