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Underway, or under way, is a nautical term describing the state of a vessel. "Way" arises when there is sufficient water flow past the rudder of a vessel that it can be steered. A vessel is said to be underway if it meets the following criteria:
- It is not aground
- It is not at anchor
- It has not been made fast to a dock, the shore, or other stationary object.
"Under weigh" is a variation, coming from folk etymology, first used in 1749. "Under way" is likely from the Dutch onderweg or Middle Dutch onderwegen (lit. "under" or "among the ways"). Weigh is also a synonym for hanging or dangling, so that the process of raising an anchor, which causes it to hang at the end of the anchor-rope or chain is called “weighing [the] anchor” which leads to confusion between weigh and way, since both are pronounced identically. 
- Merriam-Webster Mobile Dictionary, 2015, entries for "under weigh" and "under way".
- "Maloney, Elbert S. Chapman Piloting And Seamanship. 65th Ed. New York: Hearst Books, 2006."