A "flattop" is a type of short haircut where the hair on the top of the head is usually standing upright and cut to form a flat-appearing deck. This deck may be level, or it may be upward or downward sloping.

What flat top means?

: something with a flat or flattened upper surface: such as. a : aircraft carrier. b : a modified crew cut.

Is flat top a word?

flattop in American English

something having a flat or level surface, platform, etc.