Meaning
To step on, crush, or press with the foot — 'yuh mash mi toe' means 'you stepped on my toe'. Extended, 'mash up' means to break, ruin, or destroy, and 'mash down' to crush or flatten. 'Mash it up' can also mean to do brilliantly (a great performance).
Examples
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Yuh mash mi foot!
You stepped on my foot!
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Di storm mash up di whole house.
The storm destroyed the whole house.
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Di artist mash up di stage last night.
The artist tore up the stage (performed brilliantly) last night.
Origin
From English 'mash' (to crush), broadened in Patois to stepping on, destroying ('mash up'), and, admiringly, excelling ('mash it up').
How it’s used
Core sense is to step on/crush; 'mash up' = destroy OR excel, decided by context. See [[mash-up]].