fem. proper name, also the name of a butterfly genus. As a name, not much used in U.S. before 1950. It appears to have been coined by Swift c. 1711 as a pseudonym for Esther Vanhomrigh, who was romantically attached to him, and composed of elements of her name. He used it in private correspondence and published it in the poem "Cadenus and Vanessa" (1713).
The name Cadenus is an anagram of Decanus; that of Vanessa is formed much in the same way, by placing the first syllable of her sir-name before her christian-name, Hessy. [William Monck Mason, "History and Antiquities of the Collegiate and Cathedral Church of St. Patrick, Near Dublin," 1820]
As the name of a genus of butterflies that includes the Red Admiral and the Painted Lady, it dates to 1808, chosen by Danish entomologist Johan Christian Fabricius (1745-1808) for unknown reasons. He has no obvious connection to Swift, and the theory that it was intended for *Phanessa, from Greek phanes "a mystical divinity in the Orphic system" does no honor to his classical learning.