Green’s Dictionary of Slang

grassy adj.

[grass n.3 ]

(US Und.) likely to become an informer.

[US]W. Burroughs Naked Lunch (1968) 180: His dossier contains three pages of monikers indicating his proclivity for cooperating with the law [...] Leary the Nark, The Lilting Leprechaun . . . Grassy Gert.