rear end

rear end idiom
  1. The back part of anything, especially a vehicle, as in There's a large dent in the rear end of the car.
  2. The buttocks, as in I'm afraid these pants don't fit my rear end. The noun rear alone has been used in both these senses, the first since the late 1700s and the second since the mid-1900s. The addition of end occurred in the first half of the 1900s.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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