Unctuousness is the quality itself — the oily over-friendliness, the flattery poured on so thickly that it stops feeling like a kindness and starts feeling like a hand in your pocket. It is the noun you use when you want to name the behaviour rather than describe a person: 'the unctuousness of the letter', 'a certain unctuousness in his manner', 'I couldn't stand the unctuousness'. Two useful things about how it behaves. It is uncountable — you have some unctuousness, not 'an unctuousness'. And it very often follows 'the ... of' or sits after a word like 'a certain', 'the sheer', 'all that', which lets a writer point at one particular performance of fake charm rather than at a whole personality. The word is formal and mostly written; in speech, a Taiwanese student would be understood better saying 'he was so fake' or 'so smarmy'. Keep the oil in mind — from Latin unctum, ointment — because that greasy picture is exactly what an English reader feels when they see the word.

諂媚、假殷勤、油腔滑調（指這種行為或特質本身，不可數）