Casual means comfortable and informal, not for offices or ceremonies. Casual clothes, casual dress, smart casual.
Smart casual is the fixed phrase for the awkward middle ground, and nobody has ever agreed what it means. Casual also means relaxed to the point of not caring: a casual attitude, casually mentioned. And casual work is employment with no guaranteed hours: casual labour, a casual contract — a serious sense with none of the ease of the others. Note casualty, which is a different word and means a person hurt or killed.
1adjectivecasualComfortable and informal, of clothes or occasions.
“The invitation said casual and everybody wore a jacket.”
2adjectiverelaxedRelaxed to the point of not seeming to care.
“He was casual about it in a way that worried everybody.”
3adjectiveguaranteedWith no guaranteed hours, of work.
“She was on a casual contract for about three years.”
casual is a quality of informality.
Latin casualis means happening by chance, from casus, a fall or an event — the same root as case and occasion. Casual clothes are named after chance rather than comfort: they are what you happen to have on.
Proto-Indo-European
*ḱh₂d-
Latin
cāsuālis — happening by chance
Middle French
casuel
English, today
casual
Built from
From Middle French casuel, from Latin cāsuālis (“happening by chance”), from Latin cāsus (“event”) (English case), from cadere (“to fall”) (whence English cadence).
Etymology from Wiktionary, CC BY-SA