A sudden reversal of position or opinion; also a type of casual sandal with a flat sole and a strap between the toes.
The word is onomatopoetic, imitating the sound the sandal makes when slapping against the heel while walking (flip-flop-flip-flop). As a political term, it dates to at least the 1960s, describing inconsistent positions.
The brilliance of 'flip-flop' is that the sound itself tells the story—it literally mimics the sandal's rhythm, making it one of English's most onomatopoetic terms, which is probably why it stuck as a political insult too.
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