chiefly US
: belonging to an established and widely accepted group or system
mainline [=mainstream] churches
mainlines; mainlined; mainlining
slang
: to inject (a drug) directly into a vein
addicts mainlining heroin
His addiction grew worse when he began mainlining.
Cite this Entry
“Mainline.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/mainline. Accessed 24 Jun. 2026.
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