flip-flop (n.) Look up flip-flop at Dictionary.com
also flip flop, "thong sandal," by 1972, imitative of the sound of walking in them (flip-flap had been used in various echoic senses, mostly echoic, since 1520s); sense of "complete reversal of direction" dates from 1900.
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]