We’ve just returned from our annual outing to Orlando so that my wife could run the Walt Disney World Marathon. (Before you ask: 4:04:16, which is her best time for a while.) When you arrive at the airport, you have to take the train from the gates to the terminal. It’s always made me laugh when the recorded announcement, in a very Walt Disney World voice, says “This train will be departing momentarily” and I feel like quipping “when’s it going to continue?” It’s the way I tell ’em.
You see, in England momentarily generally means for a moment, briefly. Yet, in the US, it also has a meaning of in a moment, soon, despite the British sense being still accepted. You are supposed to work out the interpretation from the context, and I guess it’s only middle-aged Englishmen in Orlando that find it weird.
Note also I said weird. It’s subjective, not objective. I am certainly not being a grammar nazi here and saying the American version is wrong in any sense. The context for the word is more than the immediate sentence, it’s also social and regional. I must say I hate (with a passion) those people who jump all over such constructions and categorically say A is right and B is wrong, whatever the A and B are. Another example for another time is less than and fewer than.
1869 A. J. Evans Vashti xi. 149 Robert is bringing her home as carefully as possible, and you may expect them momentarily.
The British sense is option 1, and dates from even earlier:
1655 Ld. Orrery Parthenissa IV. ii. vi. 540 The offended God, who to make those Centinels sleepe eternally, that would not momentarily, sends down Mercury.
There are even two other definitions, as you might have surmised: at the moment, instantly, and then at every moment, moment by moment. Both of these definitions are tagged “Rare”. An example citation of the second rare definition is by Charlotte Brontë:
1847 C. Brontë Jane Eyre II. i. 1 During the early part of the morning, I momentarily expected his coming.
So, whenever I arrive in Orlando, I am momentarily disconcerted by the announcement, but the real meaning momentarily asserts itself and I find myself worrying about the luggage making it to the carousel.
Sting - Englishman in New York
(from ...Nothing Like the Sun)