Getting Under Weigh with Nautical Terms
English has many colorful expressions taken from the world of sailing, from "above board" (above the deck, or in plain view) to the "bitter end" (the very end of an anchor rope, tied to the bitts, posts mounted on the ship's deck).
The term "underway," also spelled "under way," was originally used to describe a ship in motion. The original spelling, "under weigh," is now obsolete. "Underway" and "under way" are both considered correct.