Green’s Dictionary of Slang

1. the mouth.

1860

Sporting Life (London) 17 Oct. 3/4: Tom sent his sinister mawley upon Bill’s beef-eater.

2. (US) a cattle rustler or poacher [his ‘appetite’ for cattle].

1958

(con. 1900–10) I. Blasingame Dakota Cowboy 183: A Matador [Ranch] man was always on the lookout for ‘beefeaters’ [DARE].