I. main ‧ line 1 /ˈmeɪnlaɪn/ BrE AmE adjective [only before noun]
accepted by or involving most people in a society SYN mainstream :
mainline politics
II. mainline 2 BrE AmE verb [intransitive and transitive] informal
to ↑ inject an illegal drug directly into your blood:
He was mainlining heroin.