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late 14c., "oily," from Old French unctueus, from Medieval Latin unctuosus "greasy," from Latin unctus "act of anointing," from past participle stem of unguere "to anoint" (see unguent).
Figurative sense of "blandly ingratiating" is first recorded 1742, perhaps in part with a literal sense, but in part a sarcastic usage from unction in the meaning "deep spiritual feeling" (1690s), such as comes from having been anointed in the rite of unction. Related: Unctuously; unctuousness.
unctuous unc·tu·ous (ŭngk'chōō-əs)
Containing or composed of oil or fat.