Where did the expression "Going Commando" come from, and why does it mean you're not wearing underwear?

It predates Friends by a few decades. From wikipedia

dates the modern usage to 1974 college campuses, where it was perhaps an outgrowth of the Vietnam War . The origins of the phrase are uncertain, with some speculating that it may refer to being "out in the open" or "ready for action." According to Engber many soldiers do go without underwear to "increase ventilation and reduce moisture." The earliest known use of the term in print is January 22, 1985 when Jim Spencer wrote in the Chicago Tribune:

"Furthermore, colored briefs are 'sleazy' and going without underwear ('going commando', as they say on campus) is simply gross".

Hope this helps,

"Where is that breeze coming from?" Channa

It is a reference to Marines, aka, Commando's (silent op's)

Because Marines must be ready on a moment's notice and one of the first to react in a extreme situation, it was joked/stated of that they must not bother to put on underwear first, or how else could they be so quick to go? (Fact or Myth of this joke?, good luck proving either!)

The reference established, going commando became a more common use, losing the military background for those not knowing the orgin of the joke or what commando's were/are; and becoming a statement of just not wearing underwear.

Go Commando Origin

That all depends on the underwear, if the underwear are too big or if they are low rise undies they may fall down or the jeans may push them down. However if you can see a majority of the crack then more likely than not she is going comando. Also if her underwear are low rise but her jeans aren't then you most definately may get a view of the crack but no peep of her undies.

The writers from Friends take credit for that one. Joey said "I'm not going to go commando in another man's fatigues."

I believe it was David Crane who said in an interview that they made it up, and it caught on.

Hmm...Wikipedia says it's been around since 1974...I don't know who to believe. David Crane seems pretty honest.

I don't know exactly why that term would mean what it does, though.