Slang or informal spelling of 'messing,' meaning joking around, making trouble, or fooling with something.
From 'mess' (from Old French 'mes', meaning portion of food) which evolved to mean disorder or trouble, plus '-ing'. Informal spelling drops the final 'g' in casual speech and writing.
Messin' captures how casual spoken English gets written down—it's the exact sound you'd hear in conversation! Linguists call this 'dropping the g' and it shows how written language lags behind how people actually talk.
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