Marvel finally revealed the answer: boxers with Iron Man's face on them ( screenrant.com )

Years after George Costanza and Jerry Seinfeld sat at Monk's debating what Tony Stark wore underneath his armor, Marvel finally revealed the answer: boxers with Iron Man's face on them.
Tony Stark's embarrassing moment in Unbeatable Squirrel Girl #48 settled Jerry and George's Seinfeld debate – with Jerry ultimately being proven right.
Unbeatable Squirrel Girl #48 provided a satisfyingly hilarious answer to the long-debated question raised by the classic '90s sitcom, within the context of the humor that was characteristic of the comic during its run.