good-day (n.)youdaoicibaDictYouDict[good-day 词源字典]
early 12c., "a fortunate day," also, generally, "good fortune;" from good (adj.) + day (n.). As a salutation in parting, haue godne day "have good day" is recorded from c. 1200; good day as a greeting is from late 14c.[good-day etymology, good-day origin, 英语词源]