Green’s Dictionary of Slang

beamy adj.1

[naut. jargon beam, the width of a ship]

(US) of a person, broad, wide, overweight (cf. broad in the beam under broad adj.).

G. Wasson notebooks in AS XLIX:1/2 (1974) 61: Beamy [...] physically broad (of a person).
[US]J. Gould Maine Lingo 3: Beamy [...] is a favorite Maine adjective for a lady with steatopygous accumulation.