the.com/forties

means The decade of a person's life from age 40 to 49, or the tenth decade of a century, often used as shorthand for a stretch of years defined by that number.

from Straightforward arithmetic dressed up as English: 'forty' comes from Old English 'feowertig,' literally 'four tens' (feower = four, tig = group of ten), and the plural '-ies/-ties' noun form that gives us 'the forties' as a label for a decade only became common usage from around the 19th century onward. No hidden story here — just counting by tens, the way English has always named its decades.