Green’s Dictionary of Slang

(drugs) to maintain one’s drugged or drunken state.

1930		North. Territory Times (Darwin) 11 July 7/3: When you tell the wife you’re (hic) as shober ash can be — / It’s a pound to half a cherry that you’re jagging on the spree.
1954		R. Mais Brother Man (1966) 104: Meets the gang in the gully, and jags up on his weed.
2001		ONDCP Street Terms 12: Jag — Keep a high going.