Meaning
A stupid, thick-headed person — a dunce or blockhead. To call someone 'lagga-head' is to say their head is empty and heavy, unable to hold sense or think straight. It is a blunt, mocking insult, close to 'dunce-head' and 'yam-head', flung at someone slow to learn or foolish in their doings. The image is of a head full of dead weight rather than brains, and it stings in a culture that prizes sharpness and 'brightness'.
Examples
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Nuh act like a lagga-head.
Don't act like a blockhead.
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Di lagga-head bwoy fail di simple test.
The dunce of a boy failed the simple test.
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Him too lagga-head fi understand.
He's too thick-headed to understand.
Origin
From 'lagga' (thought to relate to 'lag' — slow, lagging behind) plus 'head', for a slow, empty-headed person.
How it’s used
A blunt insult about intelligence. 'Lagga' alone can carry the same meaning.