The Collaborative International Dictionary of English v.0.48:

Chum \Chum\ (ch[u^]m), n. [Perh. a contraction fr. comrade or
   chamber fellow: cf. also AS. cuma a comer, guest.]
   A roommate, especially in a college or university; an old and
   intimate friend.
   [1913 Webster]

   New chum, a recent immigrant. [Australia]
      [Webster 1913 Suppl.]