The Collaborative International Dictionary of English v.0.48:

Encumber \En*cum"ber\, v. t. [imp. & p. p. Encumbered; p. pr.
   & vb. n. Encumbering.] [F. encombrer; pref. en- (L. in) +
   OF. combrer to hinder. See Cumber, and cf. Incumber.]
   [Written also incumber.]
   1. To impede the motion or action of, as with a burden; to
      retard with something superfluous; to weigh down; to
      obstruct or embarrass; as, his movements were encumbered
      by his mantle; his mind is encumbered with useless
      learning.
      [1913 Webster]

            Not encumbered with any notable inconvenience.
                                                  --Hooker.
      [1913 Webster]

   2. To load with debts, or other legal claims; as, to encumber
      an estate with mortgages.

   Syn: To load; clog; oppress; overload; embarrass; perplex;
        hinder; retard; obstruct; check; block.
        [1913 Webster]

WordNet (r) 3.0 (2006):

encumber
    v 1: hold back [syn: restrain, encumber, cumber,
         constrain]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "encumber":
   add, adjoin, affix, agglutinate, annex, append, attach,
   bear hard upon, block, burden, burthen, charge, clog, complicate,
   conjoin, cramp, cripple, cumber, decorate, discommode, embarrass,
   enmesh, ensnarl, entangle, entoil, entrammel, entrap, entwine,
   fetter, freight, glue on, hamper, hamstring, handicap, hinder,
   hitch on, hobble, impede, incommode, inconvenience, infix, involve,
   join with, lade, lame, lime, load, lumber, net, obstruct, oppress,
   ornament, overburden, overload, overtax, overweight, paste on,
   plus, postfix, prefix, press down, press hard upon, put with,
   rest hard upon, retard, saddle, saddle with, shackle, slap on,
   slow down, snarl, strain, subjoin, suffix, superadd, superpose,
   tack on, tag, tag on, tangle, tax, toil, trammel, unite with,
   weigh, weigh down, weigh heavy on, weigh on, weigh upon, weight