beyond (one's) depth
1. In water that is too deep for one to swim in. The lifeguard had to save the girl who had gotten beyond her depth in the pool.
2. Advanced beyond one's ability to understand or make sense of the subject matter or task at hand. The stuff they're teaching in this advanced math class is beyond my depth.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
beyond one's depth
1. Lit. in water that is too deep. Sally swam out from the beach until she was beyond her depth. Jane swam out to get her even though it was beyond her depth, too.
2. Fig. beyond one's understanding or capabilities. I'm beyond my depth in calculus class. Poor John was involved in a problem that was really beyond his depth.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.