purely and simply an alternative to "fuck". the term most likely came out of the austin, tx area in the mid 90's when austin uber alles experienced its new sexual revolution and casual sex became commonplace. fifteen, then, probably refers to the amount of time a good casual fuck takes- 15 minutes. perhaps the reference is to amount of positions.

Fifteen is a slang word used by white people to inconspicuously refer to a black person. Rumor is that it originated because 15 is the number on the black crayon in a box of crayola's.

an age that suks sixteens the year we all wanna be
duh
fifteen suks

v. the act of coitus (slang)

Another, lesser name, for rapper 50 Cent.

the fifteen pounds that many girls lose between their engagement and their wedding.

Pen Fifteen, translated to pen15, then translated from leet, means penis. Used by sofisticated nerd everywhere