What does "go off the deep end" mean?
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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.
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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.
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