off the deep end, to go

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.
See also: deep, end, off

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).
See also: deep, go, off