WordNet (r) 3.0 (2006):

sit back
    v 1: settle into a comfortable sitting position [syn: sit
         back, take it easy]
    2: be inactive or indifferent while something is happening;
       "Don't just sit by while your rights are violated!" [syn:
       sit by, sit back]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "sit back":
   be a sideliner, be still, coast, delay, do nothing, drift,
   hang fire, hibernate, idle, lie dormant, not budge, not stir, rest,
   sit it out, stagnate, vegetate, wait and see, watch and wait