"animal nipple," or, contemptuously, "the human female breast," originally with reference to suckling, 1520s, origin obscure, perhaps related to Swedish dagga, Danish dægge "to suckle."
Entries linking to dug
c. 1200, diggen, "to make a ditch or other excavation," a word of uncertain origin, perhaps related to dike and ditch, either via Anglo-French diguer, from Old French digue "dike" (which is ultimately from Proto-Germanic *dīk-, from PIE root *dheigw- "to stick, fix") or directly from an unrecorded Old English verb. The older native words were deolfan (see delve), grafan (see grave (v.)).
Transitive meanings "form by excavation, make by digging," also "obtain or remove by excavation" are from late 14c.; figurative sense of "discover by effort or search" is from early 15c. Meaning "to penetrate" is from mid-15c.; transitive sense of "cause to penetrate, thrust or force in" is by 1885.
In 19c. U.S. student slang it meant "study hard, give much time to study" (1827); the 20c. slang sense of "understand" is recorded by 1934 in African-American vernacular. Both probably are based on the notion of "excavate." A slightly varied sense of "appreciate" emerged by 1939. The strong past participle dug appeared in 16c. but is not etymological.
"excellent," slang from 1897 (often ironical),perhaps from duckie as a term of endearment (by 1853). Rev. Palmer ["Folk-Etymology," 1882] finds the use of duck as a term of endearment "identical with Danish dukke, a baby or puppet (Wolff), Ger. docke, a doll or puppet, Shetland duckie, a doll or little girl ...," and thinks it probably is not a metaphoric use of the water-bird word, or related to the much earlier slang or dialectal 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 historical rumor, did not have three of them], which perhaps is from dug (n.).