From Longman Dictionary of Contemporary Englishflattopflat‧top /ˈflæt-tɒp $ -tɑːp/ noun [countable] DCBa type of hair style that is very short and looks flat on top
Examples from the Corpus
flattop• The little guy's flattop is combed back too much, and the face is missing those character-telling wrinkles.• Two destroyers come alongside the flattop to take off crew, but it is too late. 