Definition of "erstwhile"

“Scarcely had Alice reached her twentieth birthday, than she gave her erstwhile fiancée [sic] his formal congé. […]”

1905, Baroness Emmuska Orczy, chapter 2, in The Disappearance of Count Collini‎

As an aftermath of the erstwhile competition between companies, 41 goods depots served the Liverpool and Birkenhead docks in 1923.

1964 November, P. F. Winding, “Re-shaping the LMR's North Western Line - 2”, in Modern Railways, page 343

Other erstwhile stalwarts are also wavering. Southampton had two of the best full-backs in the league last season but Ryan Bertrand has been below par this season and Cédric Soares made an uncharacteristic lapse that led to Stoke’s winning goal in Southampton’s last outing.

2017 October 14, Paul Doyle, “Mauricio Pellegrino yet to find attacking solution for stuttering Southampton”, in the Guardian‎

If you're for one of my erstwhile opponents, that's O.K. Just don't work too hard.

1999 November 1, Frank Bruni, quoting George W. Bush, “For Bush, an Adjustable Speech Of Tested Themes and Phrases”, in New York Times‎