Old English gemǣd(e)d 'maddened', participial form related to gemād 'mad', of Germanic origin.

• In English mad has always meant ‘insane’. In extreme cases a person can be as mad as a hatter or as mad as a March hare. The comparison with hatters has a sound scientific basis: in the past some hatters really did become mentally ill. Felt hats were made from fur, and one of the processes in their manufacture involved brushing a solution of mercurous nitrate on to the fur to make the fibres mat together. As a result of inhaling the mercury fumes some hat-makers suffered from mercury poisoning, which can produce symptoms such as confused speech, hallucinations, and loss of memory. The phrase was around in the 1830s, but from 1865 it was popularized by the Mad Hatter, one of the characters in Lewis Carroll's Alice's Adventures in Wonderland. As mad as a March hare arose from the excitable behaviour of hares at the beginning of the breeding season.

  ‘Mad, bad, and dangerous to know’ was how Lady Caroline Lamb described the poet Lord Byron after their first meeting at a ball in 1812. Byron was a dashing figure whose name gave rise to the adjective Byronic for a man who is alluringly dark, mysterious, and moody. ‘ Mad Dogs and Englishmen / Go out in the midday sun’ is the beginning of a 1931 song by the English dramatist, actor, and composer Noël Coward. The word madding is a rather poetic way of saying ‘acting madly’. It is most familiar through the phrase far from the madding crowd, ‘private or secluded’. Many will associate it with the title of one of Thomas Hardy's classic novels, but Hardy took the title from a line in Thomas Gray's poem ‘Elegy Written in a Country Church-Yard’, published in 1751: ‘Far from the madding crowd's ignoble strife’. Mad scientists have been with us since the 1940s, but mad cow disease for bovine spongiform encephalopathy only since the 1980s.