WordNet (r) 3.0 (2006):

cop out
    v 1: choose not to do something, as out of fear of failing; "She
         copped out when she was supposed to get into the hang
         glider" [syn: opt out, cop out]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "cop out":
   abatement of differences, abstain, abstention, accommodate,
   accommodation, adjust, adjustment, alibi, anythingarianism,
   arrangement, bargain, cash in, check out, compose, composition,
   compound, compromise, concession, conk, cop a plea, croak, deal,
   desertion of principle, die, drop, duck responsibility,
   duck the issue, escape without penalty, evade,
   evade responsibility, evade the issue, evasion,
   evasion of responsibility, excuse, fence-sitting,
   forsake the world, get away with, get by, get off, get off cheap,
   give and take, give way, give-and-take, giving way, go,
   go at liberty, go fifty-fifty, go free, go into retirement,
   go scot free, impartiality, independence, kick in, kick off,
   lame excuse, likely story, live alone, live apart,
   live in retirement, make a deal, make an adjustment,
   make concessions, meet halfway, mugwumpery, mugwumpism,
   mutual concession, neutralism, neutrality, nonalignment,
   noncommitment, noninvolvement, nonpartisanism, nothingarianism,
   out, peg out, play politics, poor excuse, reach a compromise,
   remain incommunicado, retire, rusticate, settle, settlement,
   sit it out, split the difference, stand neuter, stay at home,
   straddle, strict neutrality, strike a balance, strike a bargain,
   surrender, take the mean, take the veil, trim, understanding,
   win freedom, yield, yielding