"type of printing with lines sloping to the right," 1610s, from Latin italicus "Italian, of Italy," from Italia (see Italy). So called because it was introduced in 1501 by Aldus Manutius, printer of Venice (who also gave his name to Aldine), and first used in his edition of Virgil, which was dedicated to Italy. As a noun, "italic type," 1670s.
[Italics] pull up the reader and tell him not to read heedlessly on, or he will miss some peculiarity in the italicized word. [Fowler]
Earlier (1570s) the word was used in English for the plain, sloping style of handwriting (opposed to gothic), and italic printing sometimes in English was called cursive (and also Aldine). Often, but not always, for emphasis; in manuscripts indicated by an underscored line. Related: Italics.
The Italic words in the Old and New Testament are those, which have no corresponding words in the original Hebrew or Greek; but are added by the translators, to complete or explain the sense. [Joseph Robertson, "An Essay on Punctuation," 1785]