1
general::
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.
American Heritage Idioms