The Collaborative International Dictionary of English v.0.48:

Amigo \A*mi"go\, n.; pl. Amigos. [Sp., fr. L. amicus.]
   A friend; -- a Spanish term applied in the Philippine Islands
   to friendly natives.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

amigo
    n 1: a friend or comrade

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "amigo":
   ace, acquaintance, associate, bedfellow, bedmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   classmate, colleague, comate, companion, company, compeer, comrade,
   confidant, confrere, consociate, consort, copartner, crony,
   familiar, fellow, fellow student, girl friend, gossip, intimate,
   mate, messmate, old crony, pal, pard, pardner, partner, playfellow,
   playmate, roommate, schoolfellow, schoolmate, shipmate,
   side partner, sidekick, teammate, workfellow, yokefellow,
   yokemate