[meyn-lahyn, -lahyn]
verb (used without object), mainlined, mainlining. Slang.
to inject a narcotic, especially heroin, directly into a vein.
to use or enjoy something without restriction: to mainline on TV movies.
verb (used with object), mainlined, mainlining. Slang.
to inject (a narcotic, especially heroin) directly into a vein.
to use, enjoy, or imbibe (something) without restriction: mainlining coffee all day long.
having a principal, established, or widely accepted position; major; mainstream: the membership of mainline churches.

1935–40, Americanism; v., adj. use of main line Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Word Origin & History

1934, from main line in Amer.Eng. slang sense "principal vein into which drugs can be injected" (1933).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
First, look at today's mainline media, a hotbed of forty-year-old thinking.
Amenities on the protected side of a mainline levee, including golf courses and
  some hotels, will remain open.
Overall, he says, evangelical churches grow in members during recessions while
  mainline ones decline.
Summary of input variables for mainline freeway segments.
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