Very late 15th cent. The meaning of doubt is a poetic metaphor. Of unknown origin, presumably from a pre-form *cina. Perhaps a back formation in Vulgar Latin from Latin cinnabris "red mercuric sulfide," but the semantics are obscure and do little to help the theory.
Chin is an apocopated form.
Romance: Western Vulgar Latin: Asturian china, Portuguese china, Galician china