Word Study
bats (root: bat)
OXFORD DICTIONARY
bats, predic.adj. sl. crazy.
Etymology
f. phr. (have) bats in the belfry: see BAT(2)
THESAURUS
bats
balmy, bananas, barmy, batty, beany, bonkers, buggy, bughouse, bugs, crackers, cuckoo, daffy, dippy, dotty, flaky, flipped, freaked-out, fruitcakey, fruity, gaga, goofy, haywire, just plain nuts, kooky, loony, loopy, nuts, nutty, off the hinges, off the track, off the wall, potty, round the bend, screwball, screwballs, screwy, slaphappy, wackyFor further exploring for "bats" in Webster Dictionary Online