1843 as the years of someone's life between 40 and 49; from 1840 as the fifth 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.