The Collaborative International Dictionary of English v.0.48:

Dissemble \Dis*sem"ble\, v. i.
   To conceal the real fact, motives, intention, or sentiments,
   under some pretense; to assume a false appearance; to act the
   hypocrite.
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         He that hateth dissembleth with his lips. --Prov. xxvi.
                                                  24.
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         He [an enemy] dissembles when he assumes an air of
         friendship.                              --C. J. Smith.
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The Collaborative International Dictionary of English v.0.48:

Dissemble \Dis*sem"ble\, v. t. [imp. & p. p. Dissembled; p.
   pr. & vb. n. Dissembling.] [OF. dissembler to be
   dissimilar; pref. dis- (L. dis-) + F. sembler to seem, L.
   simulare to simulate; cf. L. dissimulare to dissemble. See
   Simulate, and cf. Dissimulate.]
   1. To hide under a false semblance or seeming; to feign
      (something) not to be what it really is; to put an untrue
      appearance upon; to disguise; to mask.
      [1913 Webster]

            Dissemble all your griefs and discontents. --Shak.
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            Perhaps it was right to dissemble your love,
            But -- why did you kick me down stairs? --J. P.
                                                  Kemble.
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   2. To put on the semblance of; to make pretense of; to
      simulate; to feign.
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            He soon dissembled a sleep.           --Tatler.

   Syn: To conceal; disguise; cloak; cover; equivocate. See
        Conceal.
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WordNet (r) 3.0 (2006):

dissemble
    v 1: make believe with the intent to deceive; "He feigned that
         he was ill"; "He shammed a headache" [syn: feign, sham,
         pretend, affect, dissemble]
    2: hide under a false appearance; "He masked his disappointment"
       [syn: dissemble, cloak, mask]
    3: behave unnaturally or affectedly; "She's just acting" [syn:
       dissemble, pretend, act]

The Devil's Dictionary (1881-1906):

DISSEMBLE, v.i.  To put a clean shirt upon the character.

    Let us dissemble.
                                                                  Adam