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1.
A random, often untraceable mystery stain that shows up after a super hard night of partying or other fun activities.

Party stains can include the following: seminal fluid, urine booze/beer/wine, drool, vomit, diarrhea, and whatever else imaginable.
1) Morty left a party stain on the hotel couch after pumping his girlfriend.

2) Moriah and Heather woke up all hungover after the huge raging party they hosted the night before. To their dismay, the place had tons of party stains and garbage strewn about the house.

Someone had smeared poop on their drapes, red wine or kool aid was all over the carpet, and vomit was placed in a cardboard box on the floor.
by jrubadub July 26, 2010
 
2.
a semen stain on the front of your boxers or jeans if going commando
aw, man i wished i had wore undies, now i got a party stain on my jeans. hope my girlfriend doesn't notice
by art-eater June 23, 2006