tough titty said the kitty

Etymology
Chosen for the rhyme. The image is of a kitten suckling at the mother cat's teat, hence the references to milk in the extended variants.

Usage notes

 * Sometimes further elaborated into various phrases involving milk: "tough titty said the kitty..."
 * but the milk sure tastes good
 * but the milk's still good
 * but the milk's damn good
 * but the milk tastes fine
 * when the milk went dry
 * when the milk wouldn't run; etc.
 * that spilled the milk