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]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "henchman":
   adherent, appendage, associate, attendant, bodyguard, buddy, buff,
   cavaliere servente, cohort, confidant, courtier, creature, crony,
   dangler, dependent, disciple, dummy, fan, figurehead, flunky,
   follower, following, gangster, gillie, goon, gunsel, hanger-on,
   hatchet man, heavy, heeler, homme de cour, hooligan, jackal,
   lackey, man, minion, myrmidon, parasite, partisan, public, puppet,
   pursuer, pursuivant, retainer, right-hand man, satellite, sectary,
   servant, shadow, sidekick, stooge, successor, supporter, sycophant,
   tagtail, tail, thug, torpedo, trainbearer, votary, ward heeler