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When a man wears tight jeans and therefore must move his penis to the side creating a "side pipe"
Man 1" These jeans are so tight"
Man 2 "I know you sure are rocking a side pipe"
Side Pipe by alycatC November 25, 2011

Side-Pipe 

When an individual of the male persuasion in an institutional circumstance (i.e. work, school, etc.) becomes sexually aroused, due to thoughts of past sexual exploits; the subsequent tightening of the groin area, due to the constriction of pants upon the hardened member, produces a penile-angular shift, which is almost perfectly horizontal in form, and points towards either the left or right. Thus the moniker, Side-Pipe.
Mosieur LePajonais: What, pray tell, is that tubular formation protruding from the side of your pantaloons?

Herr Schmidt: Why, my good man, tis my side-pipe!
Side-Pipe by ZIJPIJP June 17, 2010
When one man stelthely approaches another man from the side and rubs his man parts against another man's side. The man then shouts"Side Pipe". This activity typicaly takes place on Saturdays to celebrate "Side Pipe Saturday" .

Background: These types of games evolved from "Man Love Monday" where heterosexual men isolated from women while working in the logging camps of North Central West Virginia playfully turned to one and other to cope with isolation and lonleiness.
While Standing in the lunch line Rich snuck up on Mike and side piped him.
side pipe by Floyd Waterdriver August 23, 2011
The act of copulating the stomatic abdominal opening of a colostomy patient.
Pete - "Bro, I heard you were trying to bang Stacy. You know she has a colostomy bag right?"

Travis - "I gives no fucks. Im'ma side pipe that slut."
Side Pipe by swimp March 9, 2015
If a girl is having sex with one guy for an extended period of time but are not dating, and he goes home for the weekend, she resorts to her side pipe. This is a guy that she has banged before, but has not lead anywhere, and she keeps him on the back burner when needed.
Yo Ivan wasn't here this weekend,so she got some serious side pipe.
Side pipe by UVM'sfinest July 5, 2016

rocking a sidepipe 

When a male wears pants so tight, the outline of of his genitalia can be seen. Hence, giving the impression that he is rocking a sidepipe.
"Johnny wears pants so tight, he's rocking a sidepipe 24/7."