In a lot of early-20th-century comedy, characters tend to replace "er" sounds with "oi" sounds. Where did this originate?
EDIT: I should note that this is specifically in American comedy.
The most famous examples that I can think of are either the Nancy comic where Sluggo says that the bait shop pays him in "woims" or Bugs Bunny pronouncing "Albuquerque" as "Alba-koi-kee". I've also seen it pop up a few times in Futurama ("this isn't a war, it's a moida" comes to mind). It only seems to appear in the middle of words (the Futurama clip above said "moida," not "moidoi," for instance). It's virtually always played for comic effect, and generally seems to be associated with characters who are some combination of lower-class and witty (though not always both). I don't think I've ever heard any modern speaker talk like this unless they're specifically referencing this kind of humor.
Given the comic angle, and the general age of the media where I've seen it used (early 20th century stuff, or later series that reference early 20th century stuff), I'm broadly assuming that it comes from vaudeville and somewhere in the New York area. Given what (admittedly little) I know about vaudeville and turn-of-the-century New York, that leads me to suspect that the origin is some combination of Yiddish and Italian. That being said, I don't actually know, and I'd imagine that Italian and Yiddish are not lacking in words that have an "er" somewhere in the middle. Is this generally the correct origin, and if so, can the sound-change be traced back any further?
On a similar angle, there's some characters (generally blue-collar Southerners, such as Scruffy the Janitor, also from Futurama) who do the opposite, by pronouncing "toilet" as "terlet". I'd be very curious to learn about that switch's origins as well.