Forty

[fawr-tee] /ˈfɔr ti/

noun, plural forties.
1.
a cardinal number, ten times four.
2.
a symbol for this number, as 40 or XL or XXXX.
3.
a set of this many persons or things.
4.
forties, the numbers, years, degrees, or the like, from 40 through 49, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature:
His office is in the West Forties. Her parents are in their forties. The temperature will be in the forties.
adjective
5.
amounting to 40 in number.
/ˈfɔːtɪ/
noun (pl) -ties
1.
the cardinal number that is the product of ten and four See also number (sense 1)
2.
a numeral, 40, XL, etc, representing this number
3.
something representing, represented by, or consisting of 40 units
determiner
4.

n.

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).