Meaning
A fool — a stupid, dull, or clueless person. The word originally named someone mute or unable to speak, and from there it came to mean the dim-witted and the easily fooled, so to call someone a 'mumu' is to say they lack sense and are simple to trick. It is a plain insult, used to belittle, and it sits with 'eediat', 'chupid', and 'dunce-head' in the well-stocked Jamaican vocabulary for foolishness, aimed at the slow-witted and the gullible.
Examples
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Nuh act like a mumu, use yuh brain.
Don't act like a fool, use your brain.
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Dem tek him fi a mumu an trick him.
They took him for a fool and tricked him.
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Yuh tan deh like a mumu an mek dem walk ova yuh.
You stand there like a dummy and let them walk over you.
Origin
From 'mum'/'mute' (unable to speak), extended to mean dumb in the sense of foolish; found across the Caribbean and West Africa.
How it’s used
An insult meaning fool/dimwit; historically tied to muteness, now purely 'stupid person'.