The Collaborative International Dictionary of English v.0.48:

Messmate \Mess"mate`\, n.
   An associate in a mess.
   [1913 Webster]

WordNet (r) 3.0 (2006):

messmate
    n 1: (nautical) an associate with whom you share meals in the
         same mess (as on a ship)

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "messmate":
   ace, amigo, associate, bedfellow, bedmate, birthmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   classmate, clubmate, colleague, comate, companion, company,
   compeer, comrade, confrere, consociate, consort, copartner,
   copemate, copesmate, couchmate, cradlemate, crony, cupmate, fellow,
   fellow student, general partner, girl friend, gossip, jailmate,
   mate, old crony, pal, pard, pardner, partner, pewmate, playfellow,
   playmate, roommate, schoolfellow, schoolmate, secret partner,
   shelfmate, shipmate, shopmate, side partner, sidekick, sidekicker,
   silent partner, sleeping partner, special partner, tablemate,
   teammate, tentmate, watchmate, waymate, workfellow, yokefellow,
   yokemate