To jump off the deep end or go off the deep end is an expression related to the area of a swimming pool where the depth of water is greatest.
Literally speaking it means to jump into a swimming pool where the water is over one’s head and one needs to be able to swim in order not to put oneself in danger.
But figuratively it means to become deeply involved (with someone or something) before one is ready, especially in matters of the heart. As in: “I think John really has jumped off the deep end in his attachment to Anne. He probably should take things a bit slower”.
The idiom to jump or go off the deep end can also be used to describe someone who is acting irrationally or heedlessly, in following their emotions or fantasies without thinking things through carefully. As in: “I know you really want to go and live in Poland, but don’t go jumping off the deep end. You have a great job here”.
A further use of the phrase describes losing one’s temper or becoming very angry. As in: “One minute we were having a perfectly reasonable discussion and the next minute she just went off the deep end!”