go commando
idiom

- to not wear any underwear

Cambridge Dictionaries

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On the surface, the expression "go commando" seems a peculiar way to describe being fully dressed but without underwear. There is some speculation that the expression may go back to the Vietnam War.

To avoid fungal infections in the hot and humid climate of the jungles of Vietnam, it was apparently common practice among American soldiers to go without underwear. An editor for Slate magazine traced the usage back to college campuses in 1974, which would correspond with the end of the Vietnam conflict.

A synonym of "to go commando" is "to go regimental" and is said to refer to the Scottish infantry regiments, whose soldiers used to wear no underpants under their kilts.

A 1996 episode of the TV sitcom Friends has been credited with introducing the term into the popular language.

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Practice OWAD in a conversation:

"I can understand why some people GO COMMANDO, especially in hot and humid countries."