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Phrase(s): jump off the deep end
Go to go off the deep end., Phrase(s): jump off the deep end (over someone or something)
Fig. to get deeply involved with someone or something. (Often refers to romantic involvement.) • Jim is about to jump off the deep end over Jane. • Jane is great, but there is no need for Jim to jump off the deep end.
McGrawhill's American Idioms And Phrasal Verbs