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2.
It is actually 2 words - under way - and was originally a nautical term meaning that a ship had begun its journey.
Likely to have been of early British Naval origin.
Over time, it has also become a description of an event having started.
Hurry if you want to see the ship leave, because it will soon be under way (not underway.)

The search for the missing child is already under way (not underway.)
by Authorwise May 03, 2011
 
1.
Underway: A US Navy term meaning that the ship has pulled out of port and is out to sea. Also refered to as "Hazed, grey, and underway."
Chief: "Well it's too late to try and get out of this det now son, We're underway."
by Brad Harris September 27, 2006