1. To become active or begin operating. We've had the building finished for days now, but we're still waiting for the power to come online. Hundreds of new factories are slated to come online over the next year, which will bring a much-needed boost to the economy.
2. To log onto the Internet, usually meaning some social communication service thereof. I'm just waiting for my dad to come online, I haven't spoken to him in weeks!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
come on lineor
go on linemainly AMERICAN
COMMON If a plan, project or product comes on line or goes on line, it starts to operate fully or be available to be used. This plane was the first to be lost since the popular model came on line in 1982. The government, faced with a major energy crisis, is eagerly waiting for another reactor to go on line. Note: You can also just say that something is on line if it is operating or available. You'll have that machine back on line by nine at the latest.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012