Concerned reader God Is My Codependent sent me a link explaining it, which I promptly lost. So I looked myself and found this definition on the BBC's site:
Cream of tartar is a by-product of winemaking; it's derived from refined tartaric acid, which forms on the inside of wine barrels, or from the whitish crystals (known as 'white diamonds') that precipitate out of some wines.
Well, that's just downright odd. And it sounds vaguely illegal.