Green’s Dictionary of Slang

1. an eccentric; one who stands outside the group norms.

1970		Current Sl. V:3 5: Cornflake, n. A rustic; an unpopular student.
1998		W. Halberg Rub of the Green 37: ‘Your roommate is a real cornflake, Kendall. [. . . .] He’s a queer one’.

2. a young male homosexual.

1972		B. Rodgers Queens’ Vernacular 55: cornflakes (kwn Southwest, late ’60s) 1. loveable, big, dumb farm boy. [...] 2. generic for the clean-cut, all-American naive youth.
1979		Maledicta III:2 221: Other terms for the young and more or less innocent are [...] pogue, pretty, tail, twinkie, (bum)boy, chit, cinede (obsolete), cornflake.