Tener mala leche

Literally: To have bad milk

Example: “No le hables antes del café, hoy tiene muy mala leche.” — Don't talk to him before his coffee; today he's like a bear with a sore head.

English equivalent: Like a bear with a sore head

Origin: Rests on an ancient belief, already discussed by Aristotle and later Saint Augustine, that a nursing infant's character was shaped by the milk it drank — not just nutritionally, but temperamentally. In medieval Spain, families wealthy enough to hire a wet nurse rather than have the mother breastfeed vetted their choice carefully, since the nurse's own health, background, and even moral character were thought to pass directly into the child through her milk. Mala leche — bad milk — meant a nurse (or a mother) whose milk had transmitted a genuinely bad temperament, and the phrase kept that sense of an ingrained, almost inherited character flaw long after the underlying medical belief was abandoned.