I had to look up "going commando" as well (the really important stuff is never listed in dictionaries
). Does anyone know how that phrase was created? It seems a bit puzzling to a foreigner.
The exact origin is uncertain, dating back to either college campuses in the US in the 70s during/post-Vietnam, or Britain in the 80s post-Falklands.
It's generally agreed to refer to the practice of soldiers (especially special forces soldiers AKA commandos) not wearing underwear for various reasons: to avoid crotch-rot and chafing, to allow ventilation and avoid moisture in hot and/or wet climates, because chances to clean your underwear on long deployments are slim, and so forth.
It became really widespread and well-known when it was used on the show Friends
in 1994 when Joey said he wouldn't "go commando in another man's fatigues
Here's everything you could ever possibly want to learn about the phrase: