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.
small-mouth
smallness
smallpox
small-time
small-town
smarm
smarmy
smart
smart aleck
smart money
smart-arse