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Old English feowertig, from feower "four" (see four) + tig "group of ten" (see -ty (1)). Cf. Old Saxon fiwartig, Old Frisian fiuwertich, Dutch veertig, Old High German fiorzug, German vierzig, Old Norse fjorir tigir, Gothic fidwor tigjus.
[T]he number 40 must have been used very frequently by Mesha's scribe as a round number. It is probably often used in that way in the Bible where it is remarkably frequent, esp. in reference to periods of days or years. ... How it came to be so used is not quite certain, but it may have originated, partly at any rate, in the idea that 40 years constituted a generation or the period at the end of which a man attains maturity, an idea common, it would seem, to the Greeks, the Israelites, and the Arabs. ["The International Standard Bible Encyclopedia," James Orr, ed., Chicago, 1915]Forty winks "short sleep" is attested from 1821, In early use associated with, and perhaps coined by, eccentric English lifestyle reformer William Kitchiner M.D. (1775-1827).