The name The Forty is given to the members of the Académie Française; also called the Forty Immortals.
Forty-five an informal name for the Jacobite rebellion of 1745.
Forty Martyrs of England and Wales the name given to a group of English and Welsh Roman Catholics canonized in 1970 as representing those martyred for their faith between 1535 and 1679. Their feast day is 25 October.
forty-niner a seeker for gold in the Californian gold rush of 1849.
forty-ninth parallel the parallel of latitude 49° north of the equator, especially as forming the boundary between Canada and the US west of the Lake of the Woods.
forty winks informal expression for a short sleep.
Forty years on are the opening words of the Harrow school song by the English schoolmaster E. E Bowen (1836–1901). The words were used by Alan Bennett as the title of a satirical comedy (1968) set in an English public school.
see also a fool at forty is a fool indeed, forties, life begins at forty, Saint Swithin's day, if thou be fair, for forty days it will remain.