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.