The Collaborative International Dictionary of English v.0.48:

crackers \crackers\ adj.
   crazy. [informal or slang]

   Syn: balmy, barmy, bats, batty, bonkers, buggy, cracked,
        daft, dotty, fruity, haywire, kooky, kookie, loco,
        loony, loopy, nuts, nutty, wacky.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

crackers
    adj 1: informal or slang terms for mentally irregular; "it used
           to drive my husband balmy" [syn: balmy, barmy,
           bats, batty, bonkers, buggy, cracked,
           crackers, daft, dotty, fruity, haywire,
           kooky, kookie, loco, loony, loopy, nuts,
           nutty, round the bend, around the bend, wacky,
           whacky]