flab [1950s] Flab was formed in the 1950s from the late 17th-century flabby, itself a form of flappy [L16th] from Middle English flap, which probably, along with its further variant flop [E17th], imitates the sound of something flapping. The slang use be in a flap about something dates from the early 20th century. Flabbergast, first mentioned in 1772 as a new piece of fashionable slang and probably an arbitrary invention, may have been modelled on flabby. Flaccid [E17th] comes from flaccus, the Latin for ‘flabby’.