go commando

go commando (also gocommando, go-commando or go commando) -- to not wear underwear.

A: "Looks like Dave's goin' commando today!"
D (in camoflauge & aviator shades) : "Hey, I'm wearing underwear today."

*To not wear underwear (the reason for this really revolves around the fact that commando missions are so intense that you supposedly shit your pants...thus if you don't wear underwear during a commando mission, you basically test yourself in the attempt to ensure that you don't crap yourself during the mission).

Its origin is elusive. The implication is that to not wear underwear is a trait attributable to commandoes, who presumably don't wear any. After all, if a commando goes behind enemy lines for weeks or months, water, food, communications and munitions - the tools of their trade and survival - are of greater importance than a change of clothes.

see also
[as naked as sb's mother bore him]