French word chiant comes from Proto-Indo-European *kakka-, and later Latin caco ((vulgar) I defecate, shit, pass excrement.)

Detailed word origin of chiant

Dictionary entry	Language	Definition
*kakka-	Proto-Indo-European (ine-pro)
caco	Latin (lat)	(vulgar) I defecate, shit, pass excrement.
chyer	Middle French (frm)	(vulgar) to shit.
cacatus	Latin (lat)
chier	French (fra)	(vulgar, slang) to shit, defecate.
chiant	French (fra)	(vulgar) fucking annoying. (vulgar) fucking boring.

Words with the same origin as chiant