Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
under way, underway, under weigh
A ship has way on it when it is in motion, and thus the idiom to get under way [or underway] means “to begin to move,” just as the idiom to be under way [or underway] is “to be moving,” regardless of whether what’s moving or beginning to move is a ship, a car, or a fund-raising campaign. The idea of weigh, as with an anchor, is an error in either use, although in the past many reputable authors have chosen that form. Under way and underway are both now Standard spellings of both adverbial and adjectival uses.
