Buttermilk is more acidic than regular milk, and traditionally it's liquid left after you churn butter into cream, rather than the liquid that comes directly out of a cow (or llama or goat or whatever).

Basically it tastes very different from milk, which makes it gross when you're expecting regular milk. The Vegemite Effect, you might say.

And fuck if I know why it was in my fridge. For a recipe, maybe?