WordNet (r) 3.0 (2006):

sit up
    v 1: not go to bed; "Don't stay up so late--you have to go to
         work tomorrow"; "We sat up all night to watch the election"
         [syn: stay up, sit up]
    2: change to an upright sitting position; "He sat up in bed"

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "sit up":
   abide, arise, await, bide, bide the issue, bristle, cock up, dally,
   dawdle, delay, dillydally, disentangle, extend, flatten, get up,
   hang about, hang around, hold everything, hold on,
   hold your horses, jump up, linger, loiter, mark time, put straight,
   ramp, rear, rear up, rectify, rise, rise up, set straight,
   sit bolt upright, sit tight, smooth, stand up, stay, stay up,
   stick around, stick up, straighten, straighten out, straighten up,
   take time, tarry, unbend, uncurl, unkink, unsnarl, upheave, uprear,
   uprise, wait, wait a minute, wait and see