Meaning
Must be; must have — a contraction of 'must be', used to express a likely conclusion or an assumption drawn from the evidence. 'Him mussi gaan' means he must have gone, and 'a mussi rain a come' means it must be about to rain. It marks a confident guess or inference, the speaker reasoning from what they can see to what is probably so, and it is among the small but useful grammatical words of the language. To say a thing 'mussi' so is to conclude, on good grounds, that it is very likely the case.
Examples
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Him nuh answer — him mussi sleep.
He's not answering — he must be asleep.
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A mussi she tek it.
It must have been her who took it.
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Yuh mussi tired after all dat work.
You must be tired after all that work.
Origin
A contraction of 'must be' → 'mussi' (also 'mussa'); marks an inferred conclusion.
How it’s used
Means 'must be / must have' — an inference. Also 'mussa'. Compare 'supuozin' (supposing).