Universal Noise of ♥♥♥♥♥♥♥. See above definition for onomatopoeic use as a sound itself.

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Urban Definition
sexual. Accronym for "Universersal Noise for ♥♥♥♥♥♥♥"
particularly heard during intercouse when the female moans but bites her lip to silence herself. "unf" heard as "uuuuhhhfff" or "ooooouuuhhhnnnnnfff"
"all that ♥♥♥♥♥ could say was unf"