Losing courage or backing out of something because you're afraid (usually 'chickening out').
From 'chicken' used as slang for a coward, likely because chickens are prey animals that flee easily. The phrase 'chicken out' became common in the 1900s.
Calling someone a 'chicken' works because chickens literally run away from danger—in fact, the game of 'Chicken' where two drivers drive toward each other uses the word perfectly, since whoever swerves first is 'chickening out' and losing the game.
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