etymology - When/where/why did "Look who it ain't/isn't" appear?

It seems to me that...

"Well! Look who it ain't!"

...is/was normally used quite dismissively, referring to a newly-arrived person of low social status, who the speaker would often then proceed to denigrate at some length to the assembled company.

I said "is/was" because I'm not even sure if people still say it. I remember it as fairly common in my youth (Southern UK in the 60s), but I don't recall hearing it lately (until I just noticed it in a 70s movie).

Does anyone know when and where the usage arose? Does it have any currency among younger speakers today? And can anyone explain why it includes negation?