The quality of being casual; an air of informality, lack of concern, or nonchalance in appearance, manner, or behavior.
From casual (informal, without concern) + -ness (abstract noun suffix); casual comes from Latin casus (chance, accident).
Casualness in fashion and behavior is often a carefully constructed performance—what appears 'effortless' usually takes considerable effort, which is why haute couture designers now charge thousands for clothes that look deliberately thrown-together and informal.
Complete word intelligence in one call. Free tier — 50 lookups/day.