The type of attire one would wear to a garden party*. Such garments include button down shirts, polo shirts, sweater vests, bow ties, khakis, etc.
Origin: Derived from "Connecticut" latin for "land of the rich and waspy" and "casual" meaning "relaxed and comfortable apparel". The term was coined by Andy Bernard, Regional Manager of the Dunder Mifflin/Sabre Company.
*Garden Party: A dignified picnic or barbecue but better. It's quieter and there are rules.