Thayer's Greek Lexicon

 1. a fellow servant, one who serves the same master with another
  1) the associate of a servant (or slave)
  2) one who with others serves (ministers to) a king
  3) a colleague of one who is Christ's servant in publishing
the gospel
  4) one who with others acknowledges the same Lord, Jesus, and
obeys his commands
  5) one who with others is subject to the same divine
authority in the Messianic economy
1e1. of angels as the fellow servants of Christians