Milk Dud

Alright listen up, buttercup. So, like, a cow is supposed to give milk, right? But this one doesn't. So instead of saying it's a "milk dud" which sounds like it didn't produce any milk, you use the word "dud" which also means something that doesn't work. It's funny 'cause it's a play on words, and you probably need a nap after trying to figure that out.