The food (or just literal "oil") sense came first, though; unctuous has always been a sensory texture word. It wasn't always a positive quality, mind you; it's just one of several normal words you could use to call something "oily" in Latin. From Etymonline:
> late 14c., "oily, having a greasy or soapy feeling when touched," 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.
(Which is to say: there are probably Bible translations from before the sarcastic personality sense developed in the 1700s, that use the word "unctuous" as an adjective for the same effect that we'd today use the word "anointed.")
It's kind of fascinating that the word "unctuous" has acquired positive connotations with respect to food, from starting off as a relatively neutral descriptor. I think it's maybe a rare reversed example of the common connotative shift that happens over time, where if there's a "high [language]" word (usually a foreign import that sounds fancy and technical at the time of its introduction) and a "low [language]" word (usually a native or much older borrowed word) for something, then the "low [language]" word will acquire negative or rude connotations to speakers of that language. E.g. the difference in propriety in English between writing "I'm eating pork" vs "I'm eating pig"; or between writing "feces" vs "dung."
> late 14c., "oily, having a greasy or soapy feeling when touched," 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.
(Which is to say: there are probably Bible translations from before the sarcastic personality sense developed in the 1700s, that use the word "unctuous" as an adjective for the same effect that we'd today use the word "anointed.")
It's kind of fascinating that the word "unctuous" has acquired positive connotations with respect to food, from starting off as a relatively neutral descriptor. I think it's maybe a rare reversed example of the common connotative shift that happens over time, where if there's a "high [language]" word (usually a foreign import that sounds fancy and technical at the time of its introduction) and a "low [language]" word (usually a native or much older borrowed word) for something, then the "low [language]" word will acquire negative or rude connotations to speakers of that language. E.g. the difference in propriety in English between writing "I'm eating pork" vs "I'm eating pig"; or between writing "feces" vs "dung."