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out of one's depth

Idioms  
  1. 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.


Example Sentences

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Being out of one’s depth is an inevitable fate for a Henry Green character.

From The New Yorker • Oct. 10, 2016

Adv. beyond one's depth, out of one's depth; over head and ears; mark twine, mark twain.

From Roget's Thesaurus of English Words and Phrases by Roget, Peter Mark