Green’s Dictionary of Slang

the buttocks, the hips; thus broad in the beam under broad adj.

1836		implied in broad in the beam under broad adj.
1876		Besant & Rice Golden Butterfly I 3: A little too long of leg, a little too narrow in the beam.
1935		H. McCoy They Shoot Horses, Don’t They? in Four Novels (1983) 23: That Alabama is corn-fed. Look at that beam.