Just why the term "Mad as a hatter"? Nonsensical milliner patter? Or is it the bite from a hat sized too tight that's putting the squeeze on grey matter? The expression derives from the fact that hats are made primarily from felt, in which fibers of wool or fur are pounded together. The felting process goes more quickly when treated with mercury, and hat makers, aka milliners, would be driven crazy from mercury poisoning. Just one of many such origins of language to be found in Loose Cannons and Red Herrings, a Book of Lost Metaphors by Robert Claiborne. Wood-engravings by Thomas Dalziel.