The term flip-flop is an onomatopoeiac word based on the sound made by the sandals when walking in them.
(Flip flops are no heel strap sandals, and although this style of sandal has been worn for centuries, the modern day flip flops have been worn in America and Britain since the 1970s. Sometimes the word is written flip-flops, and flipflops.
They are called thongs in Australia, jandals (originally a trademarked name derived from "Japanese sandals") in New Zealand, slops in South Africa, and tsinelas in the Philippines.)