Green’s Dictionary of Slang

(UK Und.) an informer who betrays a large number of important fellow criminals, thus helping solve many hitherto unresolved crimes.

1984		‘Derek Raymond’ He Died with His Eyes Open 72: There was Williamson, you remember, the supergrass; you smashed both his legs with an iron bar.
1990		D. Campbell That Was Business, This Is Personal 3: The supergrass — the informer prepared to give evidence against his former associates.
2000		J.J. Connolly Layer Cake 216: You don’t want to believe that your sugar daddy’s been a fucking supergrass for years.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 181/2: supergrass n. 1 an informer, a nark.
2005		L. Redhead Rubdown [ebook] I’m sneaking around videoing her for her stuffed-shirt father and feeling like a supergrass.