To inject drugs directly into a vein, or to adopt something as a core belief or practice without question.
Compound of 'main' (primary) and 'line' (vein); became slang in drug culture by the early 20th century, and extended metaphorically to mean 'make something central to one's life or beliefs.'
The term 'mainline' reflects how drug culture influenced English slang—it entered broader use during the 1960s counterculture and was later metaphorically adopted by business and politics ('mainline Republican') without most people realizing its dangerous origins.
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