They pledged to love each other forever and a day.
Origin
1660-70; orig. phrase for ever
British Dictionary definitions for forever and a day
forever
/fɔːˈrɛvə; fə-/
adverb
1.
Also for ever. without end; everlastingly; eternally
2.
at all times; incessantly
3.
(informal) for a very long time: he went on speaking forever
noun
4.
(as object) (informal) a very long time: it took him forever to reply
5.
…forever!, an exclamation expressing support or loyalty: Scotland forever!
Usage note
Forever and for ever can both be used to say that something is without end. For all other meanings, forever is the preferred form
Word Origin and History for forever and a day
forever
adv.
late 14c., for ever; from for + ever. One word from late 17c.
Idioms and Phrases with forever and a day
forever and a day
.
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 ]