with no problems or difficulties
"put the plans into effect quickly and smoothly"
"despite of some mishaps, everything went swimmingly"
in a smooth and diplomatic manner
"`And now,' he said smoothly, `we will continue the conversation'"
in a smooth manner
"The car accelerated smoothly without any jerking movements."
The word smoothly comes from Middle English smoþely, which was formed by adding the suffix -ly to the adjective smooth. It traveled directly into modern English with the same meaning of moving without roughness or interruption.