Green’s Dictionary of Slang

mainline adj.

[mainline n.]

1. (US prison) pertaining to the main prison population (and the rules to which it is subjected).

[US]‘Goat’ Laven Rough Stuff 181: They get their mates to furnish them with work [...] where they can get some food which is not the ‘main line chuck’ (regulation prison food).
[US]D. Lamson We Who Are About to Die 92: Mainline chow is another matter.
[US]M. Braly On the Yard (2002) 251: No reason for you to eat mainline [...] You never ate mainline in Tracy, did you?
[US]E. Bunker Mr Blue 151: The condemned, who were far fewer and more quickly executed back then, were fed much better than the mainline convicts.

2. (orig. US drugs) pertaining to narcotics use and addiction.

[US]H. Gold Man Who Was Not With It (1965) 278: He didn’t have to sell mainline sugar in addition.
[US]‘Lou Rand’ Gay Detective (2003) 79: You can get anything in that joint from a piece of trade to a main line shot.
[US]E. Grogan Ringolevio 40: He had given himself his wings – his first mainline fix.