Use “underway” or “under way” as an adverb?

Question

Is it proper to use underway as an adverb? Or should under way be used?

Merriam-Webster defines underway as an adjective and under way as an adverb. The Cambridge Advanced Learner's Dictionary & Thesaurus makes no distinction.

I think this might be a split between British English and American English. Or at least using underway as an adverb is becoming more popular.

Answer 1

Which British English references? The Oxford English Dictionary comments of under way, 'now frequently as one word'. There is a separate entry for underway, and it contains several British citations from the twentieth century.

Answer 2

Dictionaries notwithstanding, underway just looks like a mistake to my eye. It gives the appearance that the writer has got it confused with underpass and underwear.

It is a similar mistake that people make with everyday. As an adverb, every day is correct because both words carry their full weight e.g. I jog every day. But the two words are joined to form an adjective as in everyday clothes. If every and day are not joined to form a compound adjective, they should at least be joined by a hyphen: every-day clothes to show that the writer has created a new adjective. (Decline in the use of the hyphen is another subject ripe for a rant!)

There is no reason to join the adverbs under way or every day with a hyphen, so it follows that there is no need to join them together at all.

Answer 3

They mean the same thing and are easily understood with or without the space; under way is the earlier form, and probably still more common, even in American English according to Google nGram.

Curiously Merriam-Webster's Dictionary of English Usage spends more time discussing the under way/under weigh difference than under way/underway. I would discourage using underweigh, which might hint at selling short measure.