If you know someone who's obsessed with how he looks and what he's wearing, you can call him a fop. If you're a fop, you make sure you're always well dressed.
A fop spends hours grooming himself in front of the mirror and spends a lot of money on nice clothes. You might also call him a "dandy" or a "clotheshorse." The word fop meant "foolish person" in the mid-1600s and was probably related to the now-obsolete verb of the same name, meaning "make a fool of." By 1670, a fop was a fool who was focused specifically on his clothing.