go off the deep end

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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.
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.
See also: deep, end, go, off
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

go off the deep end

 and jump off the deep end 
1. Lit. to jump into a swimming pool where the water is over one's head and one needs to be able to swim You are still only learning to swim. Are you ready to go off the deep end? He jumped off the deep end where he would make a bigger splash.
2. Fig. to become deeply involved (with someone or something) before one is ready. (Applies especially to falling in love.) Look at the way Bill is looking at Sally. I think he's about to go off the deep end.
3. Fig. to act irrationally, following one's emotions or fantasies. Now, John, I know you really want to go to Australia, but don't go jumping off the deep end. It isn't all perfect there.
See also: deep, end, go, off
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

go off the deep end

1. Become unduly excited, overwrought, or angry. For example, When he heard about John's smashing into his car, he went off the deep end. [c. 1920] Also see in deep, def. 2.
2. Be irrationally carried away, act irresponsibly or heedlessly. For example, Just because you like her looks doesn't mean you should go off the deep end and propose. [c. 1920] In both of these colloquial usages deep end alludes to the deep end of a swimming pool.
See also: deep, end, go, off
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

go off the deep end

mainly AMERICAN
1. If someone goes off the deep end, they start to behave in a crazy or very extreme way. Ray gives a chilling performance as the seemingly nice cop who goes off the deep end and starts terrorising a couple. At first they thought that I'd gone off the deep end and had lost my mind.
2. If someone goes off the deep end, they become very angry. My dad went off the deep end when he found out what I'd done. Note: The deep end is the end of a swimming pool where the water is deepest.
See also: deep, end, go, off
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

go off (or go in off) the deep end

give way immediately to anger or emotion. informal
This expression refers to the deep end of a swimming pool, where the diving board is located. In the USA the phrase has also developed the meaning ‘go mad’, but in either sense the underlying idea is of a sudden explosive loss of self-control.
See also: deep, end, go, off
Farlex Partner Idioms Dictionary © Farlex 2017

go off the ˈdeep end

(informal) suddenly become very angry or emotional: Don’t tell your father that you lost the money — he’ll just go off the deep end.
See also: deep, end, go, off
Farlex Partner Idioms Dictionary © Farlex 2017

go off the deep end

in. to do or experience something in the extreme: to fall madly in love, to go crazy, to commit suicide, to fly into a rage, etc. I saw what he had done, and I just went off the deep end. I was in a blind rage and didn’t know what I was doing.
See also: deep, end, go, off
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.

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, to
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
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References in periodicals archive ?
I'm pretty chilled about it, but she has gone off the deep end and now our sex life totally revolves around her fertile days.
The source added to America's Star magazine: "Brad is looking for a script about channeling, but Angelina is afraid that if he tries to bring it into the mainstream, people will really think he's gone off the deep end." (ANI)
Navy research funding appears to have gone off the deep end: The Navy has been sinking serious money into the possibility of building future warships out of glass.
The Prix Gumption award to Constantin's Bernd Eichinger for sticking to his guns when most of the German film biz thought he'd gone off the deep end (of production) on "Manitou's Shoe." He supported filmmaker Michael Herbig, and they fiddled, recut and reshot the biggest -- and funniest -- German film success stow in decades.
Added to that, there was some tale about the latest sport to come under the match-fixing spotlight (probably cricket; it usually is these days), a drug scandal and a match report from the previous night in which some not-usually outspoken manager had gone off the deep end, criticising some clueless referee.
It was, rather, an economy gone off the deep end. In Wood's words: "It was the twentieth century's best example of the dictum of Charles Mackay, the celebrated nineteenth-century historian of speculative manias, who observed that men think in herds, go mad in herds, but recover their senses one by one."
Damon has officially gone off the deep end, something that Elena (Nina Dobrev) won't be happy about when she wakes up.
Last night, the source claimed: "Stefani used to be so happy and full of life but lately I'm shocked by how much she has changed - she has gone off the deep end.
With "Across the Universe," though, she has gone off the deep end, and the result is a mess that might be the camp classic of 2007.