Definition of mainlines:
(adj) :
Normal, principal or standard.
(adj) :
(rail transport) Of or pertaining to the principal route or line of a railway.
(n) :
(rail transport) The principal route or line of a railway.
(v) :
(slang) To inject (a drug) directly into a vein.
(n) :
(slang) A principal vein into which a drug can be injected.
(v) :
(computing, transitive) To integrate (code, etc.) into the main repository for a software project, rather than separate forks.