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1.
The male act of tucking a penis behind his legs just below his ass. Often used by transvestite males to hide penis.
Watch out, that dude's no chick. He's just got a nasty tuck under and some nice fake tits.
by The Doctor February 14, 2005
 
2.
The uncomfortable condition of having your balls and sometimes penis tucked under your inos. Often caused by poorly designed seats.
The seats at the MCC computer lab always give me a bad case of tuck under.
by Forrest Klump March 12, 2009