square peg in a round hole

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.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.