noun, plural forties.
a cardinal number, ten times four.
a symbol for this number, as 40 or XL or XXXX.
a set of this many persons or things.
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.
amounting to 40 in number.

before 950; Middle English fourti, Old English fēowertig (cognate with Old Frisian fiuwertich, Old High German fiorzug, German vierzig). See four, -ty1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
forty (ˈfɔːtɪ)
n , pl -ties
1.  See also number the cardinal number that is the product of ten and four
2.  a numeral, 40, XL, etc, representing this number
3.  something representing, represented by, or consisting of 40 units
4.  a.  amounting to forty: forty thieves
 b.  (as pronoun): there were forty in the herd
[Old English fēowertig]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

O.E. feowertig, from feower "four" + tig "group of ten" (see -ty (1)). Roaring Forties are rough parts of the ocean between 40 and 50 degrees latitude. Forty winks "short sleep" is attested from 1828.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Forty years ago the trouble there amounted to a territorial dispute between two
  fairly secular tribes.
Forty years ago this month the lunar surface reverberated with life for the
  first time.
The revolution in medicine that spans those forty years in nearly exponential
  ways, and continues going forward.
Forty years ago, researchers developed a programming language that would become
  a brilliant educational tool.
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