Definitions by Some Fresh Money

Jack the mainline 

Doing heroin intravenously in the style of Robbie Williams. See also rock the mainline.

From Robbie Williams' "Rudebox":

"Ok then check the tan line
Make your body shape
Like you're stood on a landmine
Call me on my mobile
Not the landline
And jack the mainline
At the same time"

Jack the mainline by Some Fresh Money September 18, 2007