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1843 as the years of someone's life between 40 and 49; 1840 as a decade of years in a given century. See forty. Also a designation applied in various places and times to certain oligarchies, ruling classes, or governing bodies.
It is well known that society in the island [Guernsey] is, or perhaps we ought to say, for many years was, divided into two sets, called respectively the Sixties and the Forties, the former composed of the old families and those allied to them, the latter of families of newly-acquired wealth and position. ["The Dublin Review," October 1877]Roaring Forties are rough parts of the ocean between 40 and 50 degrees latitude.
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).