Green’s Dictionary of Slang

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

1899–1910		G. Wasson notebooks in AS XLIX:1/2 (1974) 61: Beamy [...] physically broad (of a person).
1975		J. Gould Maine Lingo 3: Beamy [...] is a favorite Maine adjective for a lady with steatopygous accumulation.