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1.
The insertion of the penis in the arm pit.
"That girl is so hott, i would Pit Fuck her if thats all i could get"
by the gentle giant September 10, 2005
 
2.
To have sexual intercourse with the armpit of women. (does not apply to the gays) Generally, copious amounts of lubrication are needed to ease the friction of the erect shaft. The woman in question must have shaved underarms to perform this act with decorum.
*thump thump thump thump thump thump thump thump thump thump*

"ah.. hun.. my arm is getting numb..."

"SHUT UP MOON CRICKET... I GOTTA FINISH THE PITFUCK"
by Fugget McDoogle December 06, 2006