If you are crestfallen, you are dark, depressed, and down in the dumps. You are in need of a pep talk, or at least a hug.
Although experts can’t agree on the origins of crestfallen, they do know it has something to do with fallen or defeated animals. Some say it is cocks, with their crested heads, that are responsible for the words’ origins. Others think it has to do with horses bowing their heads (touching their heads to the crest of their chests) when bested in battle. No matter which animal gave us the word, today’s humans are crestfallen when they are in low spirits.
Thanks to Vocabulary.com