Knock the whey...

Anyone familiar with the term "Knock (or slap) the whey outa you"? It looks like it has to do with some kind of farming activity.

Well, you make full-cream milk into butter by agitating it vigorously(churning it with a paddle, or rocking it) until the solid butter separates from the whey. Perhaps that's the metaphor. Then again, whey is a pale watery white colour; a traditional description of a wan feeble-looking person is "whey-faced". Perhaps the allusion is to slapping this pale person's face till it's a healthy red. (VSD)