>Start a new job or activity without being prepared for it.
>To take drastic action or react to something in an irrational or crazy way
This idiom alludes to the deep end of the pool where the water is the deepest.
>He is jumping in at the deep end, acting as strike bowler in his first match this year on the Australian pitches.
>Don’t jump off the deep end and move from this apartment just because you saw one little mouse.