D defs.my
Entry 1 sense Webster, 1913

mainline

/(mān"līn`)/ · main·line · IPA /ˈmeɪnˌlaɪn/
01 v. t. to inject (illegal drugs) into the vein; as, he preferred to mainline heroin rather than sniff coke.
  1. 1.
    to inject (illegal drugs) into the vein; as, he preferred to mainline heroin rather than sniff coke.