mettle (n.)youdaoicibaDictYouDict[mettle 词源字典]
1580s, variant spelling of metal, both forms used interchangeably (by Shakespeare and others) in the literal sense and in the figurative one of "stuff of which a person is made" (1550s) until the spellings and senses diverged early 18c.[mettle etymology, mettle origin, 英语词源]