1520s, "in a smooth manner, blandly," from smooth (adj.) + -ly (2). Meaning "without impediment or complications" is from 1660s.
put the plans into effect quickly and smoothly
`And now,' he said smoothly, `we will continue the conversation'
smolder
smooch
smoochy
smooth
smoothie
smoothly
smoothness
smorgasbord
smote
smother
smoulder
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