means The quality of being even, free from bumps, friction, or interruption — whether a surface, a sound, a motion, or a social moment.

from From 'smooth,' an Old English word ('smōth') for a surface without roughness, plus the suffix '-ness,' which English happily bolts onto adjectives to make them into qualities you can name. The deeper roots of 'smooth' are murky — its earlier history is uncertain, with no confident relatives in the sibling Germanic languages, which is unusual for so old and ordinary a word.