[potentially NSFW] Word origin question. When and why did the word 'commando' start to mean 'not wearing underwear'?

I don’t know, but I do know that in Vietnam soldiers learned not to wear undershorts to allow the maximum circulation of air. It is extraordinary how many varieties of rot and fungus are endemic in a warm humid jungle environment, and a confined dark humid space, well enough said.

As per Wikipedia -
The origins of the phrase “go commando” are uncertain, with some speculating that it may refer to being “out in the open” or “ready for action”. Slate magazine’s Daniel Engber dates the modern usage to USA college campuses circa 1974, where it was perhaps associated with an old practice in the army in Vietnam War, who were reputed to go without underwear to “increase ventilation and reduce moisture.

Also called “free-balling” or “free-buffing” for men and women respectively.

A couple of interesting theories from Urban Dictionary:

1. Named so because your junk is going in alone, and without support, much like a commando.

2. The term is theorized to be related to the much earlier term “going regimental”, which refers to wearing the kilt military style, that is, without underwear.

And one other little interesting tidbit: It also caught on to other countries such as Chile where they would say, “andar a lo gringo” which means to go about as the Americans.

@rojo – the first one (alone without support) make the most sense to me

Can’t help much, but I’ll try. When a person doesn’t wear underwear they “go commando.” The word “commando” only means a soldier trained in specific combat.
I’ve heard it said that Navy Seals, and probably other commando forces, don’t wear underwear because the sand on the beach gets into your underwear and causes chafing so they opt to “go commando.”