Know how to use "fewer" and "less"? Find out.
Old English sinder "dross of iron, slag," from Proto-Germanic *sendra- "slag" (cf. 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" (cf. 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 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.