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1.
to masterbate. pumping the pipe originates from the ancient art of placing a hand over ones shaft and moving the hand in a linear motion making sure to pull back the foreskin exposing the urethra ready for ejaculating the semen.

A popular misconception of this phrase is doing the same hand motion whilst urinating to give the false impression of a constant stream of clear semen but infact its just piss.
"Dude this morning I missed breakfast and was starving in class so I had to run to the toilet and pump the pipe into my mouth"
by Munchkin Whore March 18, 2010