Definition of bats in US English:

bats

adjective

dated, informal
  • predicative (of a person) crazy; insane.

    • ‘She was bats and contrary, but she seemed to be self-aware and have a genuine sense of humour about her own battiness and contrariness.’
    • ‘But he was bats and incomprehensible before he took ill.’
    • ‘He was bats for buying in the face of a housing downturn.’
    • ‘I think the kid and I knew she was bats but she was a hoot to be with.’
    • ‘Either that, or she was bats.’
    • ‘We realize in short order that she's bats, and that simple observation steals a great deal of power from the proceedings.’
    • ‘I have in fact stated a number or times that I think he's bats.’
    • ‘I know she's good at getting press (partially because she's really good at marketing her fair-weather femininity), but it's obvious that she's bats.’
    severely mentally ill, mentally ill, insane, mad, certifiable, deranged, demented, of unsound mind, out of one's mind, not in one's right mind, not together, crazed, maniac, maniacal, lunatic, unbalanced, unhinged, unstable, disturbed, distracted, stark mad, manic, frenzied, raving, distraught, frantic, hysterical, delirious, mad as a hatter, mad as a march hare
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Origin

Early 20th century: from the phrase have bats in the belfry (see bat).

Pronunciation

bats

/bæts//bats/