mainliner

[ meyn-lahy-ner, -lahy- ]
/ ˈmeɪnˌlaɪ nər, -ˈlaɪ- /

noun

Slang. a person who mainlines.
a person who belongs to a mainline group.

Origin of mainliner

An Americanism dating back to 1930–35; mainline + -er1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019