Pants specifically worn for standing or dancing when going out. If seated while wearing the Stand Up Pants, your muffin top and/or jelly rolls become much more visible. These pants also may be very tight, or the button may come undone upon the act of sitting.
Hince the name "Stand Up Pants."
Could also be used as an excuse to not sit down with a guy who is a turn off.
Girl 1: I wish I could go talk to the guy sitting at the bar, but I'm wearing my Stand Up Pants!
Girl 2: Oh, that's too bad!
Guy: Do you wanna sit and talk?
Girl: Oh no, I'm wearing my Stand Up Pants. Sorry.