Based on the search results, a "winter arc" seems to be a term used in fitness culture to refer to a period of time during the winter months where individuals focus on bulking up and gaining muscle mass. The term is often used interchangeably with "bear mode". It is also referred to as "The Gym Winter Arc". The term does not seem to have a specific definition or origin, but rather is used colloquially to describe a period of time where individuals focus on their fitness goals during the winter months.