"self-named, so-called, would-be," 1752 (in Chesterfield), French, from soi "oneself" (from Latin se, see se-) + present participle of dire "to say" (from Latin dicere "speak, tell, say," from PIE root *deik- "to show," also "pronounce solemnly").
word-forming element in words of Latin origin, "apart, away," from Latin se-, collateral form of sed- "without, apart, aside," probably originally "by one's self, on one's own," and related to sed, Latin reflexive pronoun (accusative and ablative), from PIE *sed-, extended form of root *s(w)e-, pronoun of the third person and reflexive (source also of German sich; see idiom).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dic- "point out, show;" Greek deiknynai "to show, to prove," dike "custom, usage;" Latin dicere "speak, tell, say," digitus "finger," Old High German zeigon, German zeigen "to show," Old English teon "to accuse," tæcan "to teach."
<a href="https://www.etymonline.com/word/soi-disant">Etymology of soi-disant by etymonline</a>
Harper, D. (n.d.). Etymology of soi-disant. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/soi-disant