The walls of this wooden house remain unpainted to this day.

unpainted 是由前缀 un-（表示"不、未"）和动词 paint（"涂漆、上色"）的过去分词 painted 组合而成的形容词。unpainted 字面意思是"未被涂漆的"，表示"保持原色、未上漆的"状态。