Mad as a hatter

This one has a genuinely grim, well-documented medical explanation. From the 18th through the 19th centuries, hat makers used mercury nitrate to treat felt in the "carroting" process, working with the toxic compound in poorly ventilated workshops for years on end. Chronic mercury exposure causes real neurological damage: tremors, slurred speech, and irritability, symptoms so associated with the trade that "mad as a hatter" and "hatter's shakes" entered common usage to describe hat workers. Lewis Carroll's Mad Hatter character in Alice in Wonderland (1865) popularised the phrase well after it was already established, rather than inventing it.