The Collaborative International Dictionary of English v.0.48:

Parasitic \Par`a*sit"ic\ (p[a^]r`[.a]*s[i^]t"[i^]k), Parasitical
\Par`a*sit"ic*al\ (p[a^]r`[.a]*s[i^]t"[i^]*kal), a. [L.
   parasiticus, Gr. ?: cf. F. parasitique.]
   [1913 Webster]
   1. Of the nature of a parasite; having the habits of a
      parasite; fawning for food or favors; sycophantic.
      "Parasitic preachers." --Milton.

   Syn: leechlike, bloodsucking.
        [1913 Webster]

   2. (Bot. & Zool.) Of or pertaining to parasites; living on,
      or deriving nourishment from, some other living animal or
      plant. See Parasite, 2 & 3.
      [1913 Webster]

   Parasitic gull, Parasitic jager. (Zool.) See Jager.
      [1913 Webster] -- Par`a*sit"ic*al*ly, adv. --
      Par`a*sit"ic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

parasitic
    adj 1: relating to or caused by parasites; "parasitic infection"
           [syn: parasitic, parasitical]
    2: of or pertaining to epenthesis [syn: epenthetic,
       parasitic]
    3: of plants or persons; having the nature or habits of a
       parasite or leech; living off another; "a wealthy class
       parasitic upon the labor of the masses"; "parasitic vines
       that strangle the trees"; "bloodsucking blackmailer"; "his
       indolent leechlike existence" [syn: parasitic,
       parasitical, leechlike, bloodsucking]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "parasitic":
   abject, accompanying, accordant, agreeing, all-devouring,
   all-engulfing, associate, associated, at one with, backscratching,
   beggarly, bloodsucking, bone-lazy, bootlicking, cadging, coacting,
   coactive, coadunate, coincident, collaborative, collective,
   collusive, combined, combining, commensal, concerted, concomitant,
   concordant, concurrent, concurring, conjoint, consilient,
   conspiratorial, cooperant, cooperative, coordinate, cowering,
   coworking, crawling, cringing, crouching, dilatory, do-nothing,
   doless, dronish, drony, easy, ecological, ecotopic, ergophobic,
   extortionate, faineant, fawning, flattering, footlicking,
   good-for-nothing, grabby, grasping, groveling, hangdog, harmonious,
   indolent, ingratiating, joint, kowtowing, laggard, lax, lazy,
   leechlike, lupine, mealymouthed, meeting, nonaggressive, obeisant,
   obsequious, on bended knee, predacious, predatory, procrastinative,
   prostrate, rapacious, raptorial, ravening, ravenous, remiss,
   saprophytic, scrounging, sharkish, shiftless, slack, slothful,
   slow, sniveling, sponging, sycophantic, symbiotic, synchronous,
   synergetic, synergic, synergistic, timeserving, toadeating,
   toadying, toadyish, truckling, unenterprising, united, uniting,
   vulturine, vulturous, wolfish, work-shy