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1.
up and under, although a term frequently used in the game of Rugby Union, is also a term used when having sexual intercourse
Good sex is achieved by having your cock up and under her buttocks you should put your hands so that you are able to pull her to you when you are pushing your cock deep into her vagina. Source - a well known sex manual which we are not allowed to advertise.
by Stias February 11, 2006
 
2.
When someone takes a shit, and defys gravity and gets the shit up and under the toilet seat.
Damn, someone just did an up and under in the bathroom.
by jwc's July 19, 2006