Bean-eater

The sense referring to Mexicans is explained by beans being a staple of Mexican cuisine. The sense referring to Bostonians came from a time when navy beans were a staple in Boston, especially as Boston baked beans. It is sometimes said to have originated in reference to immigrants there, but in usage it more often has referred to Boston Brahmins just as much as other Bostonians, and the assertion may well be only an ill-founded assumption.