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when you are jacking off and someone pulls into the driveway right as you are blowing a load
"dude i had to make a jizz run when my girlfriend pulled up last night!!"

"dude me to!!!"

by mr_kirch February 28, 2008
After the physical act of copulation, if the male ejaculate is allowed to spill inside the female cervix, she will most likely run or walk (at a brisk pace) to the lavatory to allow said ejaculate to drain from her nethers.
Last night my missus went for a Jizzrun but didn't make it, so I made her clean the floor...

With her undies.
by Thunderbox69 August 04, 2013