Also, beyond one's depth. Outside one's understanding or competence, as in He was out of his depth in that advanced calculus class, or The conductor realized that playing the fugue at the right tempo was beyond their depth. This expression alludes to being in water so deep that one might sink. [c. 1600] Also see over one's head.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.