The adjective unctuous has a few meanings:
- Characterized by affected, exaggerated, or insincere earnestness
- Having the quality or characteristics of oil or ointment
- Containing or composed of oil or fat
- Abundant in organic materials; soft and rich
The noun unction has a few meanings of its own, including “an act of anointing, esp. as a medical treatment or religious rite.”
All in one word, the meaning has moved from oil to the religious ceremony of anointment, and then to exaggerated earnestness, which might be “a sarcastic usage from unction in the meaning ‘deep spiritual feeling’”, as pointed out by the Online Etymology Dictionary.
I found it interesting that unction has a sincere meaning, while unctuous completely mocks its parent word! Unction can mean “the manifestation of spiritual or religious inspiration”, but unctuous has no similar meaning listed anywhere.