Superman’s costume has gone through many changes over the years. Originally based on a circus strongman’s tights (who wore trunks outside of their costumes like Superman’s famous “red underwear”), the suit later became the basis for many modern superhero costumes, including Batman's. Later still, the famous “S” emblem was retconned into being the family crest of Kal El’s Kryptonian bloodline, with the Man of Steel movies regularly emphasizing that it stood for “hope.”
Humorously, however, Clark Kent actually hated wearing his costume and was humiliated that he had to wear his underwear outside of his pants. This comedic take on Superman’s costume was showcased in Geoff Johns, Gary Frank, and Jon Sibal’s retelling of Superman’s early life in Superman: Secret Origin. The version incorporated more elements of Superman’s Silver Age adventures into his modern history, along with a few tongue-in-cheek observations about how a young Clark Kent might really react to running around Smallville in colored tights and a cape.
Superman: Secret Origin establishes that Clark began developing his super powers early on, exhibiting both super strength and invulnerability as a child. As he entered his early teens, he started developing heat vision, which required him to wear a set of special Kryptonian glasses in order to stop the rays from accidentally setting things on fire. Although Clark felt frustrated that his strength and heat vision kept him from being like other kids, he changed his mind when he saved Lana Lang from a tornado – and discovered his ability to fly in the process.
Telling his parents he wanted to continue using his powers to help people, Clark gained some unexpected (and initially unwelcome) aid from his mother. Deciding that her son needed to wear something much more durable if he was going to use his super powers to save others, Martha Kent showed him the invulnerable red, blue, and yellow blankets from Clark’s rocket ship and had her son use his heat vision to help her cut and sew them into a special suit. Informing Clark that she wanted him to embrace his Kyptonian heritage, Martha based the suit’s design on images she saw of the clothes Kryptonians were wearing in the holograms projected by the crystals from Clark’s ship. She saw the “S” symbol on Jor-El and felt it might be his family crest or his name. And because many Kryptonian fashions included capes and trunks on the outside of their pants, Martha incorporated those into Clark’s first Superboy costume as well.
Clark, however, was horrified by how Martha’s “adorable” costume looked on him and promised that “today is the last time I ever wear this.” Despite his negative reaction, its indestructible qualities proved handy and he reluctantly started wearing it on his rescue missions – but instead of revealing himself to the world as Superboy, he operated in secret, making the people of Smallville think they had a secret “angel” watching over them as he disappeared in a blur of red and blue.
When Jonathan Kent remarked that Clark was doing a good job of keeping out of sight, Clark responded, “Of course I’m staying out of sight, Pa. Look what I’m wearing.” Later, he grumbled that as soon as he got the chance, he would put the underwear on the inside of his suit, prompting his mother to ask him if he was remembering to wear underwear under his costume – and if it was clean. However, Clark gained a whole new appreciation for his Superboy costume when he received a visit from the Legion of Superheroes – a group of teenage heroes from the 30th century. Informing Clark that he was their inspiration, the Legion took him to the future where he saw that all the cool superhero kids were wearing costumes like his, essentially making him a trendsetter. While he hadn't officially adopted his Superman identity yet, the entire Legion knew what he was destined to do and treated him like a rock star.
Superboy ended up journeying to the future on a regular basis and finally began operating in public since everyone in the future already knew who he really was. This liberated Clark and he began gaining a new sense of confidence in his abilities and a greater pride in his costume’s design. He even started wearing his costume underneath his clothes instead of hiding it in his backpack so that he could always be prepared to fly into action – and by the time he was in his twenties, he had no problem revealing himself to the world as Superman. Years later, Superman did finally eliminate the red underwear on the outside of his costume, opting instead to wear a suit of Kryptonian armor and later a more streamlined version of his red-blue-and-yellow costume, sans trunks. However, his respect for the original design and his Kryptonian heritage remained. When his son Jon saw the original suit hanging in the Fortress of Solitude, he laughed at the idea that his father had once worn his underwear outside of his pants, prompting Clark to inform Jon that “it’s not underwear” and educate his son on the fundamentals of Kryptonian fashion.
Later still, Superman would return to his classic look after giving his wife Lois Lane his trunk-less suit to wear on an outer space adventure with their son Jon. Letting her know that he’d “find something” to wear instead of his new suit, Superman wound up going back to his original costume and proudly displayed his red trunks on his outfit once more. It might be a source of mockery among some fans and even be considered a “dated’ look by certain filmmakers, but after over eighty years, Superman’s red, blue, and yellow costume has become an iconic look that has set the standard for all other superhero suits. Even if Clark Kent himself used to hate it.
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