Meaning
A madman or crazy person — used both of someone genuinely mentally ill and, as an insult, of anyone behaving wildly, recklessly, or irrationally. 'Him a gwaan like a lunatic' means he is carrying on like a crazy person, out of control. Blunt and everyday, the word runs from real madness to plain foolish, dangerous conduct, and it sits with 'head nuh good' and 'mad' in the vocabulary of the deranged and the wild, flung at anyone whose behaviour has gone right off the rails.
Examples
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Di way him drive, him a pure lunatic.
The way he drives, he's a total lunatic.
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Nuh act like no lunatic inna public.
Don't act like a madman in public.
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Di lunatic a chat to himself.
The madman is talking to himself.
Origin
From English 'lunatic' (from Latin 'luna', moon — madness once blamed on the moon); a common word for a madman or an insult for wild behaviour.
How it’s used
A madman, or an insult for wild behaviour. Compare 'head nuh good', 'mad'.