1
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms