'flat-top' originates from English, a compound of 'flat' + 'top', where 'flat' meant 'having a level or even surface' and 'top' meant 'the highest point or upper surface'.

'flat-top' developed as a descriptive compound in English; in the early 20th century the form 'flat-top' or 'flattop' became used specifically for a military-style haircut, and 'flattop' later emerged as naval slang for an aircraft carrier.

Initially it simply described something with a flat upper surface; over time the term specialized to denote a particular short hairstyle and, by slang extension, an aircraft carrier.