Old English sinder "dross of iron, slag," from Proto-Germanic *sendra- "slag" (source also of 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" (source also of 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 after a fire has gone out" (16c.).
Geological sense "coarse ash thrown out by volcanoes" is from 1774; cinder cone, formed around a volcano by successive eruptions of ash, is recorded from 1849. Related: Cinders.