Chapfallen is an adjective that means dejected or dispirited.
A person or animal’s chaps refers to their jaw or the fleshy skin covering their jaw. It is basically the same word as ‘chops,’ as in, ‘Tyler licked his chops.’ If someone’s chops or chaps have fallen, it means that person is depressed or uninspired.
I didn’t understand the chapfallen look on the coach’s face until I looked up at the scoreboard. With the team losing forty-five to nothing, it made perfect sense for Coach Grant to be so dejected.