GO OFF (or GO IN OFF) THE DEEP END (informal) This expression refers to the deep end of a swimming pool…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.
© Farlex Partner Idioms Dictionary 2017
We have a joke in my family: that I went off the deep end a long time ago.
The ‘deep end’ was the steep drop off one side of our porch, at this rundown farmhouse we used to live in when I was a child. Just like in an inground pool, there was a ‘shallow’ end and a ‘deep’ end to our porch. The shallow end rose only a foot and a half off the ground, the other rose about four.
When I was around four years old, I was riding my tricycle back and forth from one end of the porch to the other. For some reason or other, I decided not to brake at the ‘deep’ end and rode right off the edge, falling those four feet and crashing to the ground below. With a cry of dismay and concern, my parents rushed to survey the damage. I lay on my back on the ground, tangled with my tricycle.
Instead of crying when I went off the ‘deep end’, my parents said I laughed.
The Writings of J.S. White 
A funny story. But how true that many people laugh as they “go off the deep end.” As other people watch very concerned, and that person has let all their cares go.
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I remember this, it scared me, but you thought it was funny. LOL
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