If someone looks crestfallen, their face and body show they are disappointed and a bit upset — as if the good feeling has dropped out of them. They might have just lost a game, been told no, or heard news they did not want. It is not a word for deep grief; it is the look of “oh… I really hoped that would go better.” The picture behind it is an old one: after a fight, a horse that has lost hangs its head, so the crest — the top of its neck, where the mane stands up — falls down onto its chest. People still use the word, mostly in stories and slightly careful speech. It is not slang, and it is a little more colourful than just sad or disappointed. The natural shape is how someone looks or seems: he looked crestfallen. Saying I am crestfallen is possible, but a bit heavier and less common.