beyond one's depth
in water that is too deep, do something that is too difficult; to be beyond one's ability
The executives appeared to be beyond their depth when they chose to pick a fight with the unions.

The apartment manager was beyond her depth in her effort to manage the apartment.

[TahlilGaran] English Idioms Dictionary ▲

beyond one's depth
adj. or adv. phr.
1. Over your head in water; in water too deep to touch bottom.
Jack wasn't a good swimmer and nearly drowned when he drifted out beyond his depth.
2. In or into something too difficult for you; beyond your understanding or ability.
Bill decided that his big brother's geometry book was beyond his depth.
Sam's father started to explain the atom bomb to Sam but he soon got beyond his depth.
When Bill played checkers against the city champion, Bill was beyond his depth.
Compare: OVER ONE'S HEAD¹.

[TahlilGaran] English Idioms Dictionary ▲