The Collaborative International Dictionary of English v.0.48:

Obsequious \Ob*se"qui*ous\, a. [L. obsequiosus, fr. obsequium
   compliance, fr. obsequi, fr. obsequi: cf. F. obs['e]quieux,
   See Obsequent, and cf. Obsequy.]
   1. Promptly obedient, or submissive, to the will of another;
      compliant; yielding to the desires of another; devoted.
      [Obs.]
      [1913 Webster]

            His servants weeping,
            Obsequious to his orders, bear him hither.
                                                  --Addison.
      [1913 Webster]

   2. Servilely or meanly attentive; compliant to excess;
      cringing; fawning; as, obsequious flatterer, parasite.
      [1913 Webster]

            There lies ever in "obsequious" at the present the
            sense of an observance which is overdone, of an
            unmanly readiness to fall in with the will of
            another.                              --Trench.
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   3. [See Obsequy.] Of or pertaining to obsequies; funereal.
      [R.] "To do obsequious sorrow." --Shak.
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   Syn: Compliant; obedient; servile. See Yielding.
        [1913 Webster]

WordNet (r) 3.0 (2006):

obsequious
    adj 1: attempting to win favor from influential people by
           flattery [syn: bootlicking, fawning, obsequious,
           sycophantic, toadyish]
    2: attentive in an ingratiating or servile manner; "obsequious
       shop assistants"

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "obsequious":
   abject, adulatory, ass-kissing, backscratching, beggarly, bland,
   blandishing, blarneying, bootlicking, brown-nosing, buttery,
   cajoling, complimentary, courtierly, courtly, cowering, crawling,
   cringing, crouching, deferential, fair-spoken, fawning,
   fine-spoken, flattering, footlicking, fulsome, groveling, gushing,
   hangdog, honey-mouthed, honey-tongued, honeyed, ingratiating,
   insincere, insinuating, low, mealy-mouthed, mealymouthed, menial,
   obeisant, oily, oily-tongued, on bended knee, parasitic, prone,
   prostrate, servile, slavish, slimy, slobbery, smarmy, smooth,
   smooth-spoken, smooth-tongued, sniveling, soapy, soft-soaping,
   sponging, submissive, subservient, sycophantic, sycophantical,
   sycophantish, timeserving, toadeating, toadying, toadyish,
   truckling, unctuous, wheedling