The Collaborative International Dictionary of English v.0.48:

Workfellow \Work"fel`low\, n.
   One engaged in the same work with another; a companion in
   work.
   [1913 Webster]

WordNet (r) 3.0 (2006):

workfellow
    n 1: an associate that one works with [syn: colleague, co-
         worker, fellow worker, workfellow]