Meaning

noun a closely cropped haircut; usually for men

noun a large warship that carries planes and has a long flat deck for takeoffs and landings

originFrom flat + top. Sense of “aircraft carrier” attributed to the message, “Scratch one flattop,” radioed during the Battle of the Coral Sea by LCDR Robert E. Dixon to the USS Lexington, whose commanding officer credited the pilot with coining the standard USN slang.

Etymology from Wiktionary, CC BY-SA 3.0