'smoothly-continued' originates from the combination of 'smoothly' and 'continued', where 'smoothly' means 'in a smooth manner' and 'continued' means 'to persist or carry on'.

'Smoothly' and 'continued' have been used separately in English since the Middle Ages, and their combination into 'smoothly-continued' is a modern construct to describe uninterrupted progression.

Initially, 'smoothly' and 'continued' were used separately, but their combination now specifically describes a seamless progression.