From Gary Martin at Phrase finder:

Quote:

'Way' doesn't mean here road or route but has the specifically nautical meaning of 'the forward progress of a ship though the water', or the wake that the ship leaves behind. Way has been used like that since at least the 17th century.

...

More confusion enters with doubts over the phrase's spelling. The term 'weigh anchor', and the fact that when ships are loaded with cargo and ready to sail they are weighed down, has lead to the phrase being written as 'under weigh'. This a common enough misspelling to have become almost standardised; so much so that, in his 1846 Nautical Dictionary, Arthur Young wrongly suggested that under weigh was in fact the correct original spelling:

"Under way, this expression, often used instead of under weigh, seems to be a convenient one for denoting that a ship or boat is making progress through the water, whether by sails or other motive power."

There seems little to justify it, but Young must have had some success in promoting that view as many prominent 19th century authors, including Thackeray, Herman Melville and Charles Dickens, all spelled the term that way - or should that be that weigh?


So 'way' is the original phrase, with a nautical intent, that was modified through usage.

tempus edax rerum