Chicken is often used to mean someone is a coward. This is exemplified in the game of chicken.
When did chicken first become a synonym of cowardice?
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According to this Quora answer on the exact same topic:
The primary uses, however, remain those that suggest weakness. Thus the early use to mean a girl or woman, which would become the more recent chick. Such chickens also suggest the 'cuteness' and 'fluffiness' of the creature.
The first example we have of chicken meaning a coward comes in 1600. It runs thus:
1600 W. Kemp Nine Days’ Wonder: It did him good to have ill words of a hoddy doddy! a hebber de hoy!, a chicken! a squib.