elective (adj.)youdaoicibaDictYouDict[elective 词源字典]
early 15c., "voluntary," from Late Latin electivus, from elect-, past participle stem of eligere "to pick out, choose" (see election). In U.S., in reference to school subjects studied at the student's choice, first recorded 1847. As a noun, from 1701.[elective etymology, elective origin, 英语词源]