How Well Do You Know English Slang?
1914, from colloquial verb smalm, smarm "to smear, bedaub" (the hair, with pomade), 1847, of unknown origin, perhaps somehow suggestive of the action. Verbal meaning "to smear with flattery" is from 1902.
The quality of something ''smarmy'': How to write pet stories, then, while skirting the swamps of smarm? (1937+)