"excellent," slang from 1897 (often ironical),perhaps from duckie as a term of endearment (early 19c.). Probably not related to much earlier slang noun meaning "a woman's breast" ["...whose pritty duckys I trust shortly to kysse," Henry VIII, c. 1536 letter to Anne Boleyn, who, contrary to rumor, did not have three of them].[ducky etymology, ducky origin, 英语词源]