Superman's costume defined the visual idea of a superhero so well that we still think of capes as the signature look of the genre 50 years after Spider-Man, the Fantastic Four, and a host of other popular heroes abandoned them for sleeker suits. But as iconic as Superman's costume is, it also has one of the most debated and dunked-on articles of clothing in the history of the medium: the trunks.
Jokes about Superman wearing his underwear on the outside have been cracked for decades, and DC even tried to get rid of the trunks with a series of costume redesigns in the comics and the movies. That said, they've been a part of his definitive look in thousands of stories, and you can still see them on lunchboxes and t-shirts that sport a "classic" Superman. There have even been attempts to justify them in the comics, like a scene in All Star Superman where Jimmy Olsen mentions that belted "overpants" were the height of fashion on Krypton.
The truth, however, is a lot closer to Earth. As the first superhero, Superman didn't have a template to draw from, so Jerry Siegel and Joe Shuster looked to other sources for inspiration, and given Clark Kent's amazing feats of strength, it's not surprising that they found their answers in the circus. The Man of Steel's look is a direct descendant of costumes worn by acrobats and strongmen, from the cape to the trunks worn over a form-fitting suit. The reasons are pretty obvious—when you're soaring through the air, the last place you want a visible rip in your costume is, well, right where you wear the trunks.
You can even see a similar style on another group of over-the-top heroes and villains that have their roots in circuses and carnivals: pro wrestlers often wear trunks over tights for the same reasons.