1. "Fancy" + "Pants": The most straightforward explanation is simply that "fancy pants" is a combination of the words "fancy" and "pants." "Fancy" originally meant "imaginative" or "extravagant," and "pants" were seen as a more fashionable garment than simpler trousers. So, "fancy pants" could have simply been a way to describe someone who dressed in a more stylish and elaborate manner, implying an air of pretension or snobbery.
2. "Fancy" + "Pants" + "Breeches": Another theory suggests that the term evolved from "fancy breeches." "Breeches" were knee-length trousers popular in the 17th and 18th centuries, and "fancy breeches" referred to a more elaborate and ornamented style. This could have then become "fancy pants" as trousers evolved and breeches fell out of fashion.
3. "Fancy" + "Pants" + "Pants": This theory speculates that the term could have originated from "fancy pants pants," with "pants" being repeated to emphasize the importance of the garment. This seems less likely, but it is an intriguing possibility.
4. "French" + "Pants": Some suggest that "fancy pants" could be a corruption of "French pants," which were also seen as stylish and fashionable during a certain period. This is less supported, but it does tie into the notion of "fancy pants" being associated with a certain social status or pretension.
Regardless of its exact origin, "fancy pants" has been used as a slang term for someone who is pretentious, overly concerned with appearances, or considered to be overly stylish and possibly out of touch. Its meaning is often humorous and playful, but can sometimes be used in a more derogatory way.