It's an emphatic exclamation. As you well know, it's just a contraction (corruption?) of Isn't it? serving in a rhetorical sense, not really a question but a statement of confirmation. I have a related question for you: I have sometimes heard the grammatical subject of the sentence repeated at the end of a sentence, in object form, for what appears to be the same reason, as a kind of emphasis. Examples (and please forgive incorrect execution): I like Marmite, me. We're going down the pub, us. My question is regards usage. Is this a regional use, found some places and not in others? Is it a generational thing used by people of a certain age? Is it a register thing indicative of level of education? Or is it just something some people do, like up-talking? ... etc.?
I've never heard of this. It sounds like something my five-year-old would say just because young kids are inherently redundant. Example: "This is the only last one?"
Also, I used the word "regional" in the thread title, not to relegate the whole of BrE as a regional dialect, but because I think perhaps these usages are regional within the scope of BrE, not a universal feature.
It probably originated regionally but due to the homogenisation of culture by media internet etc its difficult to nail down now, and its more likely to be found in the working / lower middle class (ie those with blue collar jobs or who live in areas where these trades once predominated You hear people say " I like stella, me" (stella artois- cheap and nasty lager) and the like, but I've never heard 'us' used like that, it would be "we're going down the pub, we are" Incidentally innit used to be a regional london thing, while 'an that' and 'like' were mostly scouse (liverpool) but all three have been picked up by a generation of wannabes so get use all over the place. One thing that is still quite regionalised is word for 'mate' - things like Bruv are used everywhere, but you get regional variations like Shag (shropshire), Charver (leeds) and so forth
I agree with moose, I heard and used it before leaving for Latvia. Also "I like that, I do." That's pretty common, using the appropriate a auxilliary verb.
Returning to add: I just saw a Portlandia episode where a woman said, "...so this one wanted a cold beer...", pointing at herself. This seems faintly slightly barely related. It also annoyed me intensely, but that's not my point.
I definitely recognise the phenomenon Wrey is talking about but I don't know anyone who uses it and I can't pin down a location. I want to say Up Norf somewhere (since I'm a Londoner, innit) and I've met people from all over the south, but not so many from the north since they haven't yet evolved beyond horse and carriage and there is no internet connection past Watford.
I don't remember ever doing this repeating of the subject in the USA. And I've not heard it in Scotland, either. This might be a regional British thing, but I don't think it's a universal British thing. As for your first example ...there's always "y'know." Or, in certain parts of Scotland "ken?" And the more recent (consequently irritating to me) and ubiquitous 'like.'
I've heard people use that phrasing in reference to other people to sort of shift blame in a joking way (similar to how if a kid's misbehaving one parent might say "well, he's your son" to the other?), so it's probably meant to be funny since she was shifting the blame onto herself. /explaining the joke ... but point being I don't think it's likely to be related. Not sure if the original non-joke phrasing is something you'd been familiar with. It is interesting how it wrapped around to appearing sort of similar, though.