also flip flop, "plastic thong beach sandal," by 1970, imitative of the sound of walking in them. Flip-flap had been used in various senses, mostly echoic or imitative of a kind of loose flapping movement, since 1520s:
Flip-flaps, a peculiar rollicking dance indulged in by costermongers, better described as the double shuffle; originally a kind of somersault. [Hotten's Slang Dictionary, 1864]
Flip-flop in the general sense of "complete reversal of direction" dates from 1900; it began to be used in electronics in the 1930s in reference to switching circuits that alternate between two states. As a verb by 1897. Flop (n.) in the sense "a turn-round, especially in politics" is from 1880.
c. 1600, "to flap," 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, "act of flopping; any action that produces the sound 'flop;' the sound itself," from flop (v.). Figurative sense of "a failure; that which is a failure" is by 1893, from the notion of a sudden break-down or collapse. Extended form flopperoo is attested by 1936. The Fosbury flop high-jumping technique (1968) is so called in reference to U.S. athlete Dick Fosbury (b. 1947), who used it to win the 1968 Olympic gold medal.
sailors' hot drink usually containing beer, brandy and sugar, 1690s, from flip (v.); so called from notion of it being "whipped up" or beaten.
1590s "to fillip, to toss with the thumb," imitative, or perhaps a thinned form of flap, or else a contraction of fillip (q.v.), which also is held to be imitative. Meaning "toss as though with the thumb" is from 1610s. Meaning "to flip a coin" (to decide something) is by 1879. Sense of "get excited" is first recorded 1950; flip (one's) lid "lose one's head, go wild" is from 1949, American English; variant flip (one's) wig attested by 1952, but the image turns up earlier in popular record reviews ["Talking Boogie. Not quite as wig-flipping as reverse side--but a wig-flipper" Billboard, Sept. 17, 1949]. Related: Flipped. Flipping (adj.) as euphemism for fucking is British slang first recorded 1911 in D.H. Lawrence. Flip side (of a gramophone record) is by 1949.
c. 1600, "talkative, nimble in talk;" 1670s, "displaying unbecoming levity," apparently an extended form of flip (v.). The ending is perhaps modeled on other adjectives in -ant or a relic of the Middle English present participle ending -inde. Shortened form flip is attested from 1847. Related: Flippantly.