The teacher's feedback on his homework was crushingly discouraging.

crushingly 是由动词 crush（压碎、粉碎）加上副词后缀 -ingly 构成的副词。crush 本义指"压碎"，引申为"彻底击败、使崩溃"的意思。 crushingly 作为副词修饰形容词 discouraging，表示"以令人崩溃的方式"，强调反馈带来的打击程度之大，可以理解为"毁灭性地、令人崩溃地"。