the word 'unctuous' traces its origins to the latin noun 'unctum', meaning 'ointment' or 'fat', which itself comes from the verb 'unguere', 'to anoint'. this literal sense of being oily or greasy passed into medieval latin as 'unctuosus'. the term entered english in the 14th century with its physical meaning, describing something rich in oil or fat. by the 18th century, the metaphorical extension developed, applying the slick, smooth quality of oil to describe overly smooth, flattering, and insincere manners, completing the journey from a physical property to a personality trait.

Structure: unctu (ointment, oil) + ous (adjective suffix) unctu is derived from latin 'unctum', meaning 'ointment' or 'fat'. ous is an english adjective-forming suffix from latin '-osus', meaning 'full of' or 'characterized by'.