Green’s Dictionary of Slang

(N.Z. prison, also mainstream boy) one who serves their sentence in the general prison population; thus v. mainstream it, to live in this manner, mainstream prison, where inmates all form part of the general population (as opposed to a segregation prison).

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 112/2: mainstreamer (also mainstream boy) n. an inmate who serves his sentence as one of the general prison population [...] mainstream it v. to serve one’s sentence as one of the general prison population [...] mainstream (also stream) adj. 1 (of an inmate) one of the general prison population 2 (of a prison) largely composed of mainstream inmates, hence a mainstream prison (as opposed to a segregation prison).