'crestfallen' originates from the combination of 'crest' and 'fallen', where 'crest' referred to the top of a helmet or head and 'fallen' indicated a downward position.

'crestfallen' changed from the literal sense of a fallen crest on a helmet, symbolizing defeat or dejection, and eventually became the modern English word 'crestfallen'.

Initially, it meant 'having a fallen crest', but over time it evolved into its current meaning of 'sad and disappointed'.