Things that fail or are unsuccessful, or shoes with loose soles that slap against the ground when walking.
From the verb 'flop' (1600s), imitative in origin, resembling the sound of something falling or slapping. Extended metaphorically to mean 'failure' in 20th-century slang.
Flip-flops are called 'flops' because of their literal sound—onomatopoeia from the 1950s—but the word perfectly captures how human failures make noise and announce themselves before we notice them.
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