Unctuous

/ˈʌŋktʃuəs/ adjective hard

Excessively smooth, flattering, and insincere in a way that feels oily or slick.

The unctuous salesman kept complimenting her taste while steering her toward the priciest option.

Memory Trick

Unctuous shares a root with 'unguent,' an oily ointment -- an unctuous person's flattery feels just as greasy and slippery as oil on skin.