mainline

UK: /ˈmeɪnlaɪn/ US: /ˈmeɪnˌlaɪn, -ˈlaɪn/

WordReference Random House Learner's Dictionary of American English © 2025

ˈmain ˈline, n. [countable]

1. Rail Transport a principal highway or railway line.
2. Slang Terms
   • Slang. a vein that can be easily reached, used for injection of a narcotic.

main•line /ˈmeɪnˌlaɪn, -ˈlaɪn/   v., -lined, -lin•ing, adj. 
v. 

1. Slang Terms to inject a narcotic directly into a vein: [no object]
   He was mainlining at $500 a day.
   [~ + [object]
   mainlined heroin for several months.

adj. 

2. having a principal or established position:
   mainline churches.

WordReference Random House Unabridged Dictionary of American English © 2025

main′ line′, 

1. Rail Transport a principal line or route of a railroad, as contrasted with a branch or secondary one.
2. Transport a principal highway.
3. Slang Terms
   • a prominent and readily accessible vein of the body that may be used for a narcotic's injection.
   • the act of mainlining.
• 1835–45

Main′ Line′, 

1. Place Names a fashionable residential district west of Philadelphia.
2. any fashionable district where socially prominent people live.
Main′-Line′, adj. 
Main′-Lin′er, n. 

main•line  (mān′līn′, -līn′), v., -lined, -lin•ing, adj. 
v.i. Slang. 

1. Slang Terms to inject a narcotic, esp. heroin, directly into a vein.
2. to use or enjoy something without restriction:
   to mainline on TV movies.

v.t. Slang. 

3. Slang Terms to inject (a narcotic, esp. heroin) directly into a vein.
4. to use, enjoy, or imbibe (something) without restriction:
   mainlining coffee all day long.

adj. 

5. having a principal, established, or widely accepted position;
   major;
   mainstream:
   the membership of mainline churches.

• verb, verbal, adjective, adjectival use of main line 1935–40, American.

Collins Concise English Dictionary © HarperCollins Publishers::

main line noun

1. the trunk route between two points, usually fed by branch lines
2. (as modifier): a main-line station
3. a main road

verb mainline

1. (intransitive) to inject a drug into a vein

adjective mainline

1. having an important position, esp having responsibility for the main areas of activity

ˈmainˌliner noun