Meaning of “mainline” in the English Dictionary

"mainline" in British English

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mainlineadjective

uk /ˈmeɪn.laɪn/ us /ˈmeɪn.laɪn/

mainlineverb [ I or T ]

uk /ˈmeɪn.laɪn/ us /ˈmeɪn.laɪn/ slang

to inject (= put into the body through a needle) drugs directly into the blood:

Several of her friends were mainlining heroin.
By now she was mainlining.

(Definition of “mainline” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)