Party Swag is when one falls asleep after becoming drunk or otherwise under the influnce of some substance at a party in those same clothes and wakes up and stil uses them. Often after sleeping over after a party one may not be able to change and doesn't go home right away.
Ex. going to go get food after waking up.
Guy 1: dude that girls checking you out, you need to go talk to her.
Guy 2: I smell like booze man, she isn't gonna dig that.
Guy 1: nonsense, it's your Party Swag
talking man, she'll know your down to party.