Green’s Dictionary of Slang

beam n.1

[note beam-ends n.; ult. naut. jargon]

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

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