Meaning
A person's manner, behaviour, or vibe — the way they carry on. 'A wah kinda flex dat?' asks what kind of behaviour is that, usually at something odd or objectionable. As a verb, 'flex' can mean to relax and take it easy ('mi jus a flex'), or to operate, move, and scheme in a certain way ('watch how him flex'). A flexible, much-used word of the newer slang, it names the whole style and conduct of a person, and reading someone's flex is reading how they move through the world.
Examples
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A wah kinda flex yuh a run?
What kind of behaviour are you on?
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Mi nuh like him flex.
I don't like his vibe / his ways.
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Wi just a flex an cool out.
We're just relaxing and taking it easy.
Origin
From English 'flex' (to bend, be flexible), extended in Jamaican and diaspora slang to a person's flexible way of moving through a situation.
How it’s used
Very common: 'a nice flex' is a good vibe; 'a dead flex' is a dull one. Distinct from the newer 'flex' meaning to show off, though that sense is spreading.