Looks like people have ran out of answers too.
^^^ This. And succinctly expresses at that.
Take your cleanest dirty pair, turn them inside out, and wear those. Then, take the next cleanest and wear them over the first. Frees up space in the suitcase, too.
Gato- that’s really ingenious of you. How long did it take you to figure that out?
(Jeez, men!)
In the future, buy so-called “travel” underwear like Ex Officio (I prefer the Give-N-Go fabric). They can be washed in the shower at night and they’ll be dry by morning easily.
If you are flying Tokyo to Chicago, you will arrive earlier than you left, so just keep wearing the ones you have on. Alternatively, if you go Chicago to Tokyo, you will be wearing the same ones for two days, and you will realize that is perfectly acceptable.
I’ll endorse the travel underwear. It works well.
I’m going to disagree on ran versus run. I can’t find a source for how you’re using it. Could you point me towards a reference for this?
1 pair of underwear will last 4 days.
Day 1: Wear as usual
Day 2: Rotate underwear 180 degrees.
Day 3: Turn underwear inside out.
Day 4: Repeat day 2 procedure.
Day 5: (Emergency Only) - Switch used underwear with a close friend.
I have run. It’s the third irregular form in the conjugation “run ran run”. As in “speak spoke spoken”, hence “I’ve spoken”, or “I’ve run”.
Wear underwear two days, no big deal.
I’ve never considered it but then, I’ve never run out of underwear. It’s the least-bulky item in your suitcase - why not bring a few spares?
No, it’s “I’ve RUN out of underwear”.
I would wear uses ones before I would put on an unwashed from the store.
^ This.
Seriously, you can’t wash a pair of underwear in the sink? You don’t even have that as an option in the poll so I’m assuming it didn’t even occur to you even if it’s the obvious solution for me.
I’m assuming you’re a guy but don’t go commando, that just sounds very uncomfortable and will possibly leave you over exposed and if you forget to zip oh lordy spare the rest of us.
YOu could go down to the laundry and pinch a new pair, j/k…
Look for a bvd dispenser in the airport if you must, lol.
Sitting in the lobby with a hat out and a sign saying “underwear for a poor packer”.
That is what the OP ended up doing, according to post #8. I assume that no, they did not think of it, which is why this poll was useful and helpful to our hapless OP. 
Glad we were of help.
Just keep wearing your existing pair. Plenty of people go hiking, camping, on deployment, etc. where they have to wear the same clothing (including underwear) for days on end without changing, and they are fine.
Being over exposed is not the worst wardrobe malfunction a guy could have Involving a zipper while going commando. PM for grisly details.
Word.