I'm American and I've heard Americans say "I beg your pardon", but like you I've always thought of it as a slightly proper (maybe WASP-y) idiom. People frequently say "excuse me", "sorry?", or "say again?". At least I do. Maybe I should get my ears checked.

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cromka 2 hours ago | parent | next [-]

They'll also commonly say "pardon me", which is a bit nicer "say again", but definitely nowhere close to "I beg your pardon" uptightness.

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tdeck 6 hours ago | parent | prev [-]

I'm familiar with the expression but if an American said that to me, I'd probably think it meant "rethink what you just said".

	▲	roarcher 4 hours ago \| parent [-]
		It depends a lot on their tone. Most of the time I've heard it, it's a quick "begpardon?", sometimes with their ear cocked towards you. When I see it in writing, I too for some reason picture an angry posh British man who is about to demand satisfaction.