Be out of (one's) depth phrase
To be in a state of affairs that is too difficult for you to handle or comprehend.
I am a monolingual person. Therefore, when he asked me in a foreign language, I was out of my depth.
At that time, I was only able to ride a bike, so when he told me to ride a motorbike, I was out of my depth.
This expression alludes to being in water so deep that one might sink. It dates from around 1600.
Children or adults often say this phrase to get their friends to join in something, especially to jump into a swimming pool (pond, lake, etc.)
“Last one in is a rotten egg!” yelled Jane, jumping into the pool.