The Collaborative International Dictionary of English v.0.48:

Henchman \Hench"man\ (h[e^]nch"man), n.; pl. -men (-men). [OE.
   hencheman, henxman; prob. fr. OE. & AS. hengest horse + E.
   man, and meaning, a groom. AS. hengest is akin to D. & G.
   hengst stallion, OHG. hengist horse, gelding.]
   An attendant; a servant; a follower. Now chiefly used as a
   political cant term.
   [1913 Webster]

WordNet (r) 3.0 (2006):

henchman
    n 1: someone who assists in a plot [syn: confederate,
         collaborator, henchman, partner in crime]