"Go off at the deep end" is an idiomatic expression that means to suddenly become angry, agitated, or irrational over a particular issue. There are several synonyms for this phrase, including fly off the handle, blow a fuse, lose one's cool, flip out, go ballistic, and have a meltdown. These phrases imply that the person is overreacting or behaving in an unreasonable manner. Another synonym is to "get bent out of shape," which suggests that the person is becoming excessively upset about something that is not worth getting upset over. Overall, these phrases describe a sudden and intense emotional reaction that is out of proportion to the situation at hand.