Yours, Etc.: Origins and Uses of 8 Sign-Offs
1610s, "not to know, to be ignorant of," from French ignorer "be unaware of," from Latin ignorare "not to know, disregard" (see ignorant). Sense of "pay no attention to" first recorded 1801 (Barnhart says "probably a dictionary word"), and not common until c.1850. Related: Ignored; ignoring.