(French). Former, of times gone by. As Ci-devant governor —i.e. once a governor, but no longer so, Ci-devant philosophers means philosophers of former days.
“The appellation of mistress put her in mind of her ci-devant abigailship.” —Jane Porter: Thaddeus of Warsaw, chap. xxi.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894