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).