A famous Welsh-ism, with no identifiable source but readily associated with the Welsh stereotype. Even the Welsh seem to readily acknowledge it and can be heard to jokingly state it in the most exaggerated accent (think 'oh boyo, how green was my valley?). If anything it is thought of far more fondly than the usual 'sheep shagger' accusations gratuitiously flung their way.
My Welsh friends tell me it's just one of those special things about living in Wales, likening it to question/statements like, "Whose car is that Nissan?"
There has to be a proper history of this saying, one that Google could not unlock.
Other Welsh-isms include 'Tidy' (as in "Check out my DVD burner" - "Tidy, mate"), 'Safe' (as in "Let's go down Wind Street to that new pub" - "Safe"), and most famously, 'You Knows It' (as in "The new MG roadster is safe as fuck" - "You knows it!"). Naturally, being Wales, 'fuckin' can be inserted in and around these expressions.
Updates: New catchphrases include "Is it?" (as in "The Gulf War is on!" - "Is it?"), "Look you now" and "I'll be there now in a minute". My girlfriend rummaged furiously through her bag searching for her phone, and exclaimed, "Agh! Where's it to?", and "Where was you going when I saw you coming back?"
More Updates: "Butt", used in a similar way to "mate", "buddy", "guy", etc. As in, "Alright, butt?". I was at a party where someone's dad said it to me, heard my Canadian accent, and immediately apologized in case I didn't understand 'butt' was not negative. Also I hear "clart" bandied around a bit, but alas have yet to get a proper definition.
Still More Updates: "I loves it tells you!", "I likes blinkin' I dos!" and, "This are how you speaks in Cardiff, like!" - all heard in satire on Red Dragon FM, 'performed' by the insufferable Jase. I've also started work at a place with a Merthyr girl from the dreaded Gurnos estate, whose accent is so thick even other Welsh natives have difficulty understanding her bizarre ranting.
Yet more updates: You can say a regular sentence and add a nice closure like "haven't I", "isn't it", "doesn't it" or similar
ONOZ more updates: "So I says to him like, I says..." and decribing people as, "that boy", as in, "That boy needs to worker harder or they'll sack him won't they."
Updatey!: Hit comedy Gavin and Stacey has raised awareness of the madness of the Valleys, with the finest quote thus far being "Where's she to now then?". I also overheard an English colleague exclaim 'I loves Cardiff' during a recent trip.