Archaic third-person singular form of 'run,' used in older English as in 'he runneth' instead of 'he runs.'
From Old English 'rinnan' or 'irnan.' The '-eth' suffix was the standard way to conjugate verbs in Early Modern English, seen in the King James Bible and Shakespeare.
The '-eth' ending was completely standard 400 years ago but sounds archaic now—language changes so fast that what's normal in one generation becomes odd in the next.
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