Kiwi (New Zealand) and Aussie (Australian) slang expressing frustration in an amusing way - as in for something that's really close but isn't quite there yet and would really annoy you because of it (is often used in Murphy's Law situations)
Sally: I only needed 1 more white shirt, but they've all sold out in my size!
Mary: Oh, "Wouldn't that just rip ya undies!"
Sally: Haha yeah - couldn't find any of them either!
Bob: I only needed 2% more to take the village over!
John: Oh! Wouldn't that just rip ya undies!
Bob: lol totally