go off the deep end

1. Literally, to jump into the deep end of a pool (as opposed to the shallow end). Don't let Sally go off the deep end—she doesn’t know how to swim. I was only allowed to go off the deep end once I'd finished two years of swimming lessons. A: "Is Billy allowed to go off the deep end?" B: "Sure. I know he's young, but he's a really strong swimmer."

2. slang To become crazy or irrational. A; "Now your father thinks the neighbors are plotting against him." B: "Wow, he's really gone off the deep end!" Whoa, man, stop yelling! I only put a tiny scratch on your car, so there's no need to go off the deep end. You can't tell people you want to start a business selling bees as pets—they'll think you've gone off the deep end.

off the deep end

slang Crazy or irrational. A; "Now your father thinks the neighbors are plotting against him." B: "Wow, he's really gone off the deep end!" Whoa, man, stop yelling! I only put a tiny scratch on your car, so there's no need to go off the deep end.

off the deep end, to go

To overreact; to let one’s emotions carry one away. The “deep end” presumably means the deep water at the end of a swimming pool, which it is rash to enter unless one can swim. The term became current in the early twentieth century. “There’s no reason for your going off the deep end,” wrote Kathleen Knight (Rendezvous with the Past, 1940).