(dated) Underpants, type of underwear worn in skin contact with the hip portion of the body, small enough to be worn invisibly under shorts. Typically refers to male, not female, underpants.
I have one clean pair of undershorts.
I have to buy more undershorts.
1934, Edgar Rice Burroughs, chapter 3, in Pirates of Venus, page 41:
It being very warm, I removed all of my clothing except my undershorts and lay down to sleep.
1969, Philip Roth, “The Jewish Blues”, in Portnoy’s Complaint, New York: Vintage, published 1994, page 47:
[…] like a mouse [I] hop frantically about on my toes, trying to clear my feet of my undershorts before anybody can peek inside, where, to my chagrin, to my bafflement, to my mortification, I always discover in the bottommost seam a pale and wispy brushstroke of my shit.
1989, Shashi Tharoor, “The Thirteenth Book: Passages through India”, in The Great Indian Novel, New York: Arcade, published 1993, page 268:
I have been debriefed—I believe that is the current expression, though I am always tempted, when I hear it, to make sure I still have my undershorts on—by the mandarins of the External Affairs Ministry.