Green’s Dictionary of Slang

cull n.2

[SE cull, to select weak animals for killing]
[1970s]

1. (US campus) a socially unacceptable person.

2. (US campus, spec. fraternity) anyone rejected for membership in a fraternity or sorority.

3. (US prison) a physically weak convict.

In compounds

cull bird (n.)

[1970s] (US campus) any woman considered socially or physically unacceptable.