WordNet (r) 3.0 (2006):

cover up
    v 1: hide from view or knowledge; "The President covered the
         fact that he bugged the offices in the White House" [syn:
         cover, cover up]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "cover up":
   act, act a part, affect, alibi, apology, apply to, assume,
   be the goat, becloud, befog, blanket, blind, block, bluff,
   camouflage, canopy, cloak, clothe, cloud, color, conceal, cope,
   counterfeit, cover, cover story, cowl, curtain, device, disguise,
   dissemble, dissimulate, distract attention from, eclipse, ensconce,
   enshroud, envelop, excuse, facade, fake, feign, feint, film,
   four-flush, front, front for, gammon, gloss, gloss over, guise,
   handle, hide, hood, keep under cover, lame excuse, lay on,
   lay over, let on, let on like, locus standi, make a pretense,
   make as if, make believe, make like, mantle, mask, muffle, obduce,
   obfuscate, obscure, occult, ostensible motive, overlay, overspread,
   play, play a part, play possum, playact, poor excuse, pretend,
   pretense, pretension, pretext, profess, protestation,
   public motive, put on, put-off, refuge, screen, scum, semblance,
   shade, sham, shield, show, shroud, simulate, slur over,
   smoke screen, spread over, stalking-horse, stratagem, subterfuge,
   superimpose, superpose, trick, varnish, veil, whitewash