WordNet (r) 3.0 (2006):

sit tight
    v 1: maintain the same position; wait it out; "Let's not make a
         decision--let's sit tight"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "sit tight":
   abide, await, be incognito, bide, bide the issue, burrow, dally,
   dawdle, delay, dillydally, disappear, drop from sight,
   go into hiding, go to ground, go underground, hang about,
   hang around, hide, hide away, hide out, hold everything, hold on,
   hold your horses, hole up, lie, lie close, lie doggo, lie hid,
   lie low, lie snug, linger, loiter, mark time, masquerade,
   play peekaboo, remain anonymous, retire from sight, sit up, stay,
   stay in hiding, stay up, stick around, take cover, take time,
   tarry, wait, wait a minute, wait and see, wear a mask