'flop' originates from imitative/onomatopoeic English, representing the sound or action of a heavy fall or slap.

'flop' appeared in English in the late 17th to early 18th century as an echoic verb meaning 'to fall or hang loosely'; by the 19th and early 20th centuries it broadened to nouns meaning 'a heavy fall' and later, in 20th-century show business, to mean 'an unsuccessful entertainment' (a 'flop').

Initially it meant 'to fall with a heavy sound' or 'to hang loosely', but over time it developed the additional sense of 'a complete failure' (especially for performances and products).