Underway, or under way, is a nautical term describing the state of a vessel which is unconstrained from horizontal translational movement relative to the water and the ground.
"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:[1] If a vessel is adrift and not being propelled by any instrument or device, it is said to be underway, not making way.
[2] "Under weigh" is a variation, coming from folk etymology, first used in 1749.
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.