Meaning

adjective unpleasantly and excessively suave or ingratiating in manner or speech

originFrom Late Middle English unctuous [and other forms], borrowed from Medieval Latin ūnctuōsus (“greasy, oily, unctuous”), from Latin ūnctum (“ointment; rich banquet; rich savoury dish”) + -ōsus (suffix meaning ‘full of; overly’ forming adjectives from nouns). Ūnctum is a noun use of the perfect passive participle of unguō (“to anoint; to smear with oil, to grease or oil”), from Proto-Indo-European…

Etymology from Wiktionary, CC BY-SA 3.0