Begin a journey or a project.
We are familiar with this little expression but, like other idioms that we absorb without question when learning the language, it doesn't make much literal sense. Why 'under'? Why 'way'? It turns out that a confusing of several seafaring words had a bearing on the coining of this nautical phrase.
'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; for example, this piece from Samuel Sturmy's Mariners Magazine, 1669:
"If you sail against a Current, if it be swifter than the Ship's way, you fall a Stern."
This usage continued into the 20th century and was also used in aviation as well as shipping. In 1911, The Times reported:
"He shut off his engine and by so doing took the 'way' off the biplane."
The term 'under sail' and 'underway' appear at first sight to be quite similar. The former seems easy to interpret, as sailing ships are literally under the sails when in motion, but what are we under in 'underway'? That is easier to understand when we know that this 'under' was originally 'on the'. Knowing that, 'on the way' makes sense. 'On the way' migrated to 'underway', probably due to the influence of the Dutch word 'onderweg', which translates into English as 'underway' but to 17th century sailors must have sounded more like 'on the way'.
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 led 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?
The word 'way' is now flourishing as the converse of 'no way', but as 'a ship's progress' it is all but defunct. The loss of the association with shipping has removed the incentive to spell the phrase 'get under weigh' and 'get underway' is now commonly used (although some still prefer to use the two-word form for 'get under way' and reserve the single 'underway for the adjectival 'the underway ship').
See other Nautical Phrases.