Green’s Dictionary of Slang

1. (US gang) a traitor.

1979

H.C. Collins Street Gangs 222: Cheeser Give information to police, traitor.

2. (US campus) one who is considered socially unacceptable.

1996

Eble Sl. and Sociability 120: During 1990–92, all of the following nouns categorized someone as not attuned to the prevailing style or priorities of UNC-CH student life: [...] cheddar, cheese, cheeseball, cheeser.