WordNet (r) 3.0 (2006):

back out
    v 1: move out of a space backwards; "He backed out of the
         driveway"
    2: make a retreat from an earlier commitment or activity; "We'll
       have to crawfish out from meeting with him"; "He backed out
       of his earlier promise"; "The aggressive investment company
       pulled in its horns" [syn: retreat, pull back, back
       out, back away, crawfish, crawfish out, pull in one's
       horns, withdraw]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "back out":
   abandon, abjure, back down, backpedal, backwater, beat a retreat,
   beg off, boggle, chicken, chicken out, climb down, crawfish out,
   cry off, deny, depart from, desert under fire, disavow, discard,
   disclaim, disengage, disown, draw back, draw off, drop out,
   eat crow, eat humble pie, evacuate, fall back, falter, forsake,
   forswear, funk, funk out, get cold feet, give ground, give place,
   go back, go back on, jettison, jilt, leave, leave behind,
   leave flat, lose courage, maroon, move back, pull back, pull out,
   quit, quit cold, recant, renege, renounce, repudiate, resile,
   retire, retract, retreat, revoke, run back, say goodbye to,
   scuttle, skedaddle, stand back, stand down, swallow, take back,
   take leave of, throw over, unsay, vacate, welsh, withdraw