American expressions Brits don’t understand

Whilst visiting the states last month, I discovered there’s a lot of things Americans don’t understand about the British (one being the difference between England, Britain and the UK). But there’s also a lot the British don’t understand about Americans.

BBC America has compiled a list of five American expressions Brits don’t understand. Here are my two favourite, with descriptions:

“Oh snap!”
Oh what? Snap? Snap what? What ARE you on about? And what purpose does this expression serve? Are you saying it to commend a smart-ass for their witty quip, or pat them on the head condescendingly for trying? Is it one of those phrases that started out as a high fiving “oh you got SERVED” and has now ended up meaning an eye-rolling “nice try, Seinfeld”? It’s just that we’d probably be quite good at saying it, if it turned out to be the latter. Eye-rolling is a British specialty, after all.

“Momentarily”
I only found out about this the other day, but it’s a good one. It seems that our two great nations have different understandings of the same word. Suppose you have friends coming over to stay, and they send a text message when they’re just around the corner, just to be sure you’re ready for them: “Hi! We’ll be there momentarily!” In America, this means “we’ll be there in a moment,” but in the UK, it means “we’ll be there FOR a moment.”

I understand both of these expressions – perhaps my love of American television is to blame? What British expressions do Americans find difficult to interpret?