The Collaborative International Dictionary of English v.0.48:

bats \bats\ (b[a^]ts), a.
   Crazy; insane; loony; demented; batty.
   [PJC]

WordNet (r) 3.0 (2006):

bats
    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]