WordNet (r) 3.0 (2006):

keep quiet
    v 1: refrain from divulging sensitive information; keep quiet
         about confidential information; "Don't tell him any secrets
         --he cannot keep his mouth shut!" [syn: keep quiet, shut
         one's mouth, keep one's mouth shut] [ant: babble,
         babble out, blab, blab out, let the cat out of the
         bag, peach, sing, spill the beans, talk, tattle]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "keep quiet":
   abide, be mute, be silent, be still, coast, forswear speech,
   freeze, keep mum, keep silent, keep still, lie still, make no sign,
   mark time, not breathe, not stir, play dumb, remain,
   remain motionless, repose, rest, say nothing, shut up, stand,
   stand fast, stand firm, stand mute, stand still, stay, stay put,
   stick, stick fast, tarry, tread water