greenhorn (n.)youdaoicibaDictYouDict[greenhorn 词源字典]
mid-15c., "horn of an animal recently killed," also "young horned animal," from green (adj.) in sense of "new, fresh, recent" + horn (n.). Applied to new soldiers from c. 1650; extended to any inexperienced person by 1680s.[greenhorn etymology, greenhorn origin, 英语词源]