Unsubstantialness is a word that is often used to describe something that lacks substance, weight, or importance. However, there are plenty of synonyms that can be used in place of this word to express the same idea. Some of these synonyms include flimsiness, insubstantiality, fragility, frailty, tenuity, and slightness. These words can be used to describe objects, ideas, or arguments that lack weight or substance. For instance, you might describe an argument as having an air of insubstantiality, or you might comment on the fragility of a piece of artwork or a set of glassware. In any case, these synonyms provide plenty of alternatives to the word unsubstantialness.