<p><strong>Verb</strong></p><ul><li>(Used without object) Slang.<ul><li>To inject a narcotic, especially heroin, directly into a vein.</li><li>To use, enjoy, or imbibe something without restriction: mainlining coffee all day long.</li></ul></li></ul><p><strong>Adjective</strong></p><ul><li>Having a principal, established, or widely accepted position; major; mainstream: the membership of mainline churches.</li></ul>