unctuous

excessively smooth, flattering, or oily in manner

Roots

-ous / -ious: full of or marked by a quality
unct / ungu: anoint, smear, oil, or ointment

Other words with the unct / ungu root

anoint: to apply oil in a ceremonial or sacred way
ointment: medicinal oil or salve for the skin
unction: anointing; smooth or excessive earnestness
unguent: soft oily substance used on skin

Unctuous behavior feels oily: too smooth to trust. Ointment and anoint give the texture before the spelling gets strange.

The unctuous salesman praised every choice before mentioning the price.

Unctuous is one of those words where the texture teaches the meaning. The spelling does not look like ointment, so keep the image instead: manners covered in too much oil, smooth, shiny, and hard to trust.