The Collaborative International Dictionary of English v.0.48:

Serf \Serf\, n. [F., fr. L. serus servant, slave; akin to
   servare to protect, preserve, observe, and perhaps
   originally, a client, a man under one's protection. Cf.
   Serve, v. t.]
   A servant or slave employed in husbandry, and in some
   countries attached to the soil and transferred with it, as
   formerly in Russia.
   [1913 Webster]

         In England, at least from the reign of Henry II, one
         only, and that the inferior species [of villeins],
         existed . . . But by the customs of France and Germany,
         persons in this abject state seem to have been called
         serfs, and distinguished from villeins, who were only
         bound to fixed payments and duties in respect of their
         lord, though, as it seems, without any legal redress if
         injured by him.                          --Hallam.
   [1913 Webster]

   Syn: Serf, Slave.

   Usage: A slave is the absolute property of his master, and
          may be sold in any way. A serf, according to the
          strict sense of the term, is one bound to work on a
          certain estate, and thus attached to the soil, and
          sold with it into the service of whoever purchases the
          land.
          [1913 Webster] Serfage

WordNet (r) 3.0 (2006):

serf
    n 1: (Middle Ages) a person who is bound to the land and owned
         by the feudal lord [syn: serf, helot, villein]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "serf":
   apple-polisher, ass-licker, backscratcher, backslapper, bondmaid,
   bondman, bondslave, bondsman, bondswoman, bootlick, bootlicker,
   brown-nose, brownie, captive, chattel, chattel slave, churl,
   clawback, concubine, courtier, creature, cringer, debt slave,
   dependent, dupe, fawner, feudatory, flatterer, flunky, follower,
   footlicker, galley slave, groveler, handshaker, hanger-on, helot,
   homager, inferior, instrument, jackal, kowtower, lackey,
   led captain, lickspit, lickspittle, liege, liege man,
   liege subject, mealymouth, minion, myrmidon, odalisque, peon,
   puppet, retainer, servant, slave, spaniel, stooge, subject,
   subordinate, suck, sycophant, theow, thrall, timeserver, toad,
   toady, tool, truckler, tufthunter, underling, understrapper,
   vassal, villein, yeoman, yes-man