cinder: [OE] Despite the similarity of form and meaning, cinder has no etymological connection with French cendre (which comes from Latin cinis ‘ashes’, and is thus related to English incinerate and cineraria, a plant so named because of the grey down on its leaves). It is a Germanic word, related to German sinter ‘deposit formed by evaporation’ (itself borrowed into English in the 18th century), and from Old English times until the 16th century was usually spelled with an initial s-; the c- is an adoption from French cendre.
The name of the fairy-tale character Cinderella is a translation and adaptation of French cendrillon, originally a generic term for any downtrodden kitchen maid who spent much of her time among the cendres of the hearth.
cinder (n.)
Old English sinder "dross of iron, slag," from Proto-Germanic *sendra- "slag" (cognates: Old Saxon sinder "slag, dross," Old Norse sindr, Middle Low German and Middle Dutch sinder, Dutch sintel, Old High German sintar, German Sinter), from PIE root *sendhro- "coagulating fluid" (cognates: Old Church Slavonic sedra "cinder").
Initial s- changed to c- under influence of unrelated French cendre "ashes," from Latin cinerem (nominative cinis) "ashes," from or related to Greek konis "dust" (see incinerate). The Latin word was contracted to *cin'rem and the -d- inserted for ease of pronunciation (compare peindre from pingere). The French word also apparently shifted the sense of the English one to "small piece of burnt coal" (16c.). Volcanic cinder cone is recorded from 1849.