mid-14c., auncyen, from Old French ancien "old, long-standing, ancient," from Vulgar Latin *anteanus, literally "from before," adjectivization of Latin ante "before, in front of, against" (from PIE *anti "against," locative singular of *ant- "front, forehead;" see ante). The parasitic -t dates from 15c. by influence of words in -ent.
Specifically, in history, "belonging to the period before the fall of the Western Roman Empire" (and contrasted with medieval and modern). In English law, "from before the Norman Conquest." Ancient of Days is from Dan. vii:9. Related: Anciently.