For a very long time, as in He's been working on that book forever and a day. This hyperbolic expression probably originated as a corruption of the now obsolete for ever and ay. Shakespeare used it in The Taming of the Shrew (4:4): “Farewell for ever and a day.” Today it is mainly a substitute for “very long time.”
[ c. 1600
Incessantly, ceaselessly, as in Will this racket never end? It's been going on forever and a day.
[ ; first half of 1900s