Third person singular of 'fit', meaning to be the right size or shape, or to be suitable. As a noun, refers to sudden attacks of illness or emotion.
From Middle English 'fitten', possibly from Old Norse 'fitja' (to knit). The medical sense of 'seizure' comes from Old English 'fitt' meaning 'conflict' or 'struggle', suggesting the body's struggle during a seizure.
The word 'fits' perfectly demonstrates English's economy - one small word covers everything from trying on clothes to describing epileptic seizures to expressing anger ('throwing a fit'). The connection is the idea of something happening suddenly or precisely!
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