Estar como una cabra

Literally: To be like a goat

Example: “Se bañó en el río en enero; está como una cabra.” — He swam in the river in January; he's mad as a hatter.

English equivalent: Mad as a hatter

Origin: Grew out of direct rural observation: Spanish farmers and shepherds, working closely with goats alongside more placid farm animals like sheep and cattle, noted how much more unpredictable and erratic goats' behavior could be by comparison — genuinely more prone to sudden, chaotic movement. Young goat kids in particular are known for near-constant frantic activity, reinforcing the association, and goats' habit of ramoneo — browsing on shrubs, brambles, and other unlikely vegetation most grazing animals avoid — added to their reputation for behaving strangely. "Estar como una cabra," comparing an eccentric or unpredictable person to a goat, grew naturally out of this everyday rural familiarity with the animal's genuinely erratic habits.