Pay the full amount on a debt or on wages, as in The car's finally paid off, or Les pays off the workers every Friday evening. [Early 1700s]
Produce a profit, as in That gamble did not pay off. [Mid-1900s]
Also, pay off an old score. Get revenge on someone for some grievance, require, as in Jerry was satisfied; he'd paid off his ex-partner when he bought him out at half-price, or Amy went out with her roommate's boyfriend, but she was paying off and old score.
Bribe, as in The owner of the bar paid off the local police so he wouldn't get in trouble for serving liquor to minors. [Colloquial; c. 1900]
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