Borrow some. Does it matter if they’re male or female? 
Yes, but those situations all occur largely out in the fresh air and/or among others who are similarly hygiene challenged. The OP is facing a long trip in the both close and closed confines of an airplane. Being fresh, clean, and unperfumed is de rigueur.
If your room has an ironing board & iron, you can significantly speed up the drying process.
I’ve had this dilemma in real life. When quickly packing for a business trip, I grabbed three pairs of underwear for a three night trip. Upon showering on the last day, I discovered that unbeknownst to me, my wife had a pair of underwear which were a similar material to my briefs but cut for a woman. I chose the wear an aired out pair option.
Why exactly are these underwear not usable for a couple of days in a row? What exactly are you doing to make using underwear unusable?
TMI: I easily wear the same underwear two days in a row. I’m not shitting in them or anything.
Exactly the scenario that occurred to me. :eek:
I’d still go commando, though.
My day-olds smell like testosterone while washed smells of dryer sheets. No I don’t shit or piss in mine either, but I definitely could pass a blind A/B test on the smell.
BTW, the sink wash worked out great. Washed them that night in hot water + bar soap, wringed 'em out, hung them on a hanger (the kind with clips) over the shower. They were 99% dry in the morning. Took the blow dryer to them for a minute. Perfect. Thanks! I don’t know why it wasn’t obvious to me to do this, but whatever, we all have blind spots.
If I was dirty enough that I wanted to wash my underwear, I’d probably wear them in the shower, as I’ve done before. But I’d probably do it before my last day: I like to have clean underwear for the trip home.
This always make me think of Longfellow …
While I agree that it certainly should be, in my experience it most definitely is not.
I roll in up in a towel and then wring the towel – I twist the whole thing up tight. Because the the structural strength of the towel, the garment inside doesn’t get stretched. I’ve done this after an emergency hand-wash for t-shirts and shorts as well as underwear. It also works for wet bathing suits.
Sometimes I intentionally plan to do some washing on a long trip. Sometimes I unexpectedly got my period.
Yeah, my day-old underwear neither feel nor smell like clean underwear. I will wear pants and shirts and bras for a few days, but not underpants or socks, if I can possibly avoid it.
I got a few pairs of these (Ex Officios) once and tried to quick dry both those and just normal underwear on a trip. The normal (reasonably priced) underwear dried; the (crazy expensive) Ex Officios didn’t. In other words, “quick dry”? my damp, uncomfortable ass.
Be careful with the blow drier. While the obvious solution is to turn it on high, it’ll overheat and turn itself off. Medium will keep it going long enough to dry your clothing.
Just for the record, it’s wrung them out, not wringed. Guess you have another blind spot! 
When a sock is damp (and not wet) I put the hair dryer air outlet inside the sock. The thing dries in like a few second.
If they’re all cotton, you’ll definitely need to blow them dry. I was in a similar situation a couple years back - and 36 hours later the damn things were still damp. I still had one clean pair that I wore… and left the other in the car during a long driving day, figuring that between the sunlight and the car’s A/C, they’d be dry enough for the following day (it worked).
If I were doing a longer trip where I needed to have as little luggage as possible, I’d probably invest in some microfiber undies - they may not dry super fast like some of the adverts say, but they’d do better than pure cotton.
I always carry a little (TSA-safe) bottle of liquid soap. I mostly use it for hand-laundry, but it also functions as hand soap or shampoo in a pinch. Before I carried that, I used the hotel’s shampoo for emergency laundry.
EEEEEEEEEEEEEEEEEWWWWWWWWWW! No, no, and definitely no.
Instead, you swing by a store and buy a package.
Sure, if you bury your face in them. Presumably the people around you will not be burying their face in your crotch on this particular day, so nobody is going to be alert to your mildly elevated level of crotch aroma if you choose to wear your underwear for a second day.
If you know for a fact that such a presumption is wrong, then yes, it’s probably wise to take the trouble to hand-wash a pair of underwear for your final travel day.
And next time, pack a couple more pairs than you think you’ll need. I even shove a spare pair in my bag for day trips because if you get stuck for two hours on a bus with broken air conditioning in July you’re not stuck walking around with swamp-crotch all day. 
PS, And a Ziplock bag to put the dirties in, of course.