Meaning
That — the Patois pronunciation of 'that', with the 'th' hardened to 'd'. 'A dat mi a talk bout' means that's what I'm talking about, and 'dat one' points out a particular thing. It also lands at the end of a sentence for emphasis, as in 'a lie dat!' — that's a lie! Among the most common everyday words, 'dat' does the plain grammatical work of the demonstrative 'that', singling out and pointing to a thing, and its emphatic sentence-final use is a characteristic turn of the language's lively, pointed speech.
Examples
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A dat mi did a seh.
That's what I was saying.
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Gi mi dat one deh.
Give me that one there.
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A lie dat!
That's a lie!
Origin
The Patois pronunciation of 'that' — 'th' hardened to 'd', the same shift as in 'dis', 'wid', 'dem'.
How it’s used
Means 'that'; the sentence-final 'dat' adds emphasis ('a fool dat!'). See [[dis]], [[wid]].