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."

Etymology
*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).

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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]