beyond one's depth

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 HEAD (1).