Something that's crushing is terribly upsetting or overwhelming. Your favorite team's crushing loss to its rival comes as a huge, disappointing surprise.
The adjective crushing can be used to describe physical devastation, like a boxer's crushing blow to his opponent's face, but it more often describes a humiliating or dismaying situation. Bad news might be a crushing blow, and being turned down for a date to the prom could feel like a crushing rejection. Crushing comes from the Old French cruissir, a variation on croissir, "to gnash, crash, or break."