excessively flattering or insincere
unctuous - insincere
If a person is ‘unctuous’ then you have to take what they say with a pinch of salt because they are usually displaying false interest or friendliness in order to get something from you. Such people tend to be quite predatory in that they will shamelessly manipulate somebody for personal gain, so this formal word has strongly negative connotations.
David’s repeated praise for his manager just sounds unctuous to me. He is clearly trying to secure himself a promotion.
I only spoke to the door-to-door salesperson for a few minutes, but that was more than enough time to be put off by his unctuousness.
The reporters behaved far too unctuously around the politician and failed to ask her any tough questions.