broad in the beam
Having broad hips or large buttocks. For example, I've grown too broad in the beam for these slacks. This expression originated in the 17th century and described the wideness of a ship. It began to be used for the human body only in the 1920s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.