WordNet (r) 3.0 (2006):

slip away
    v 1: leave furtively and stealthily; "The lecture was boring and
         many students slipped out when the instructor turned
         towards the blackboard" [syn: slip away, steal away,
         sneak away, sneak off, sneak out]
    2: pass by; "three years elapsed" [syn: elapse, lapse,
       pass, slip by, glide by, slip away, go by, slide
       by, go along]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "slip away":
   bow out, creep out of, depart, disappear, duck out, escape, exit,
   find a loophole, go on furlough, go on leave, leave the scene,
   mooch off, shuffle out of, skulk away, slide off, slide out of,
   slink off, slip off, slip out, slip out of, slip through,
   slither off, sneak off, sneak out, sneak out of, sneak through,
   squirm out of, steal away, take leave, worm out of,
   wriggle out of