gaily (adj.)youdaoicibaDictYouDict[gaily 词源字典]
also gayly, "with mirth and frolic," late 14c., from Middle English gai (see gay) + -ly (2). "The spelling gaily is the more common, and is supported by the only existing analogy, that of daily" [OED].[gaily etymology, gaily origin, 英语词源]