The Winter Arc is a term that has different meanings depending on the context. According to Urban Dictionary, it can refer to the following:

• A time where every man must face the mental and physical challenges of winter. A time to put your head down and get things done.
• The arc in which a man bulks up by gaining as much muscle as possible during winter while wearing a hoodie/baggy clothes to later cut the excess fat in time.

In a Reddit post, it is mentioned that the Winter Arc refers to "Bear mode", which is the best time to bulk up without worrying about looking fat as you have multiple layers of clothing on. It is a time period where you do most of the bulking since you can eat more in winters. However, it is also mentioned that it can lead to uncontrolled bulk, where you gain a slight bit of muscle with a whole lot of fat.

In a YouTube video, the Winter Arc is referred to as the Winter Bulk, which is a period where people focus on gaining muscle mass and strength during the winter months. The video shows a fitness enthusiast documenting his journey of bulking up during the winter season.

In summary, the Winter Arc can refer to a period of time during winter where people focus on gaining muscle mass and strength, or it can refer to a time where people face the mental and physical challenges of winter to get things done.