Why is the ninth month called September?
c.1600, probably a variant of flap with a duller, heavier sound. Sense of "fall or drop heavily" is 1836, that of "collapse, fail" is 1919; though the figurative noun sense of "a failure" is recorded from 1893. Related: Flopped; flopping.
1823, in the literal sense, from flop (v.). Figurative use by 1893.