When you wear your work clothes, going out clothes, or any clothes you wore during the day, to bed. Often happens to adults with multiple children due to lack of sleep and laziness. 24 hour outfit may also happen to people that are drunk or don't care.
Jim: Wake up honey
Nancy: Aghh i'm so tired
Jim: Hey, why are you still wearing your suit from work?
Nancy: Must have been a 24 hour outfit
Daughter: Are you going to bed
Dad: Yeah, I am. Why?
Daughter: Because your not wearing your pajamas, your wearing your jeans and t-shirt!
Dad: Yep. It's a 24 hour outfit.