Green’s Dictionary of Slang

1. a toady, a sycophant.

1971		F.J. Hardy Outcasts of Foolgarah (1975) 10: For every honest lurk man like Chilla there has to be a letter of the law regulation enforcer, a lurk detector like Brown Tongue Parker.
1984		Partridge DSUE (8th edn) 141/1: C.20.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 31/2: brown tongue n. 1 an inmate who curries favour obsequiously with prison authorities.

2. (N.Z. prison) as ext. of sense 1, i.e. such an inmate must have ingratiated themself with the authorities, an inmate of minimum security.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 31/2: brown tongue 2 an inmate on minimum security.