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.