unctuous \UNGK-choo-us\ (adjective) - 1 : Of the nature or quality of an unguent or ointment; fatty; oily; greasy. 2 : Having a smooth, greasy feel, as certain minerals. 3 : Insincerely or excessively suave or ingratiating in manner or speech; marked by a false or smug earnestness or agreeableness.
"Peter approached Linna wearing a smile so unctuous it seemed about to slide right off his face."
Unctuous is from Medieval Latin unctuosus, from Latin unctus, "anointed, besmeared, greasy," past participle of unguere, "to anoint, to besmear."