WordNet (r) 3.0 (2006):

let on
    v 1: make known to the public information that was previously
         known only to a few people or that was meant to be kept a
         secret; "The auction house would not disclose the price at
         which the van Gogh had sold"; "The actress won't reveal how
         old she is"; "bring out the truth"; "he broke the news to
         her"; "unwrap the evidence in the murder case" [syn:
         unwrap, disclose, let on, bring out, reveal,
         discover, expose, divulge, break, give away, let
         out]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "let on":
   acknowledge, act, act a part, admit, admit everything, affect,
   allow, assume, avow, betray, bluff, come clean, concede, confess,
   cop a plea, counterfeit, cover up, disclose, dissemble,
   dissimulate, divulge, fake, feign, four-flush, gammon, give away,
   grant, let on like, let out, make a pretense, make as if,
   make believe, make like, open up, out with it, own, own up, play,
   play a part, play possum, playact, plead guilty, pretend, profess,
   put on, sham, simulate, spill, spill it, spit it out, tell,
   tell all, tell the truth, uncover