Meaning
Can't; cannot — the negative of 'can', with the distinctive Patois 'cy' glide. 'Mi cyaa do it' means I can't do it, and 'yuh cyaa manage' is you can't manage. Note that 'can' itself is likewise often said 'cyan', so that ability and inability can sound close, and tone and context tell them apart. Among the most common everyday words, 'cyaa' does the plain work of denying that something is possible, marking what a person is unable to do.
Examples
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Mi cyaa manage all a dat by miself.
I can't manage all of that by myself.
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Yuh cyaa tell mi nutten.
You can't tell me anything.
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Him cyaa come today.
He can't come today.
Origin
From 'can't', with the palatal 'cy' glide characteristic of Jamaican pronunciation of 'ca-' words (car → cyar, can't → cyaa).
How it’s used
The everyday 'can't'. The palatal glide ('cy') is a hallmark of Jamaican speech.