sexual. Accronym for "Universersal Noise for Fucking"
particularly heard during intercouse when the female moans but bites her lip to silence herself. "unf" heard as "uuuuhhhfff" or "ooooouuuhhhnnnnnfff"
For all of you vulgar people it can also mean the abbreviation for Unified National Fine (as in thread of a mechanical device that has a spiral groove cut in it for holding and fastening things together, most times called a "screw" that has no connotations to vulgarity.
When purchasing screws be sure to specify the thread type UNF as opposed to UNC