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Phrase(s): a square peg in a round hole
Fig. someone who is uncomfortable or who does not belong in a particular situation. (Also the cliché: trying to fit a square peg into a round hole, trying to combine two things that do not belong or fit together.) • I feel like a square peg in a round hole at my office. Everyone else there seems so ambitious, competitive, and dedicated to the work, but I just want to make a living. • Trying to teach me math is like trying to fit a square peg into a round hole. I’m convinced my brain is not built right to understand algebra.
McGrawhill's American Idioms And Phrasal Verbs
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Also, round peg in a square hole. A misfit, especially a person unsuited for a position or activity. For example, Ruth doesn't have the finesse for this job; she's a round peg in a square hole. This idiom, with its graphic image of something that cannot fit, dates from about 1800.
American Heritage Idioms