When you've been steadily partaking of alcohol, hash cookies, bongs etc in the seated position, only to realise upon standing up that you're too goddamn wasted for your legs to be capable of fulfilling their normal function. Derived from the practice of painting on (or using decals to represent) non-functional elements to decorate cars (eg bullet holes), model aircraft (eg instrument panels) and so forth.
I thought I was still OK until I went to take a dump with my legs painted on and ended up shitting myself in the hallway.
Girl: Let's take this to the bedroom.
Guy: AllRIGHT! Yeah baby -- oh shit, your legs are painted on, I'm gonna have to drag your ass.
I can't believe you survived that bike ride with your legs painted on, man, you puked all over my garage and went to sleep on a wheel ramp.