In full swing means fully underway, very active, or operating at peak energy. People use it for events, seasons, projects, conversations, and any situation that has moved beyond the beginning stage.
The phrase most likely grew from the idea of something swinging with full force. A full swing suggests strong, complete movement, so the expression came to describe activity that is lively, busy, and already well developed. Earlier uses were also connected with dancing and energetic motion, which helps explain the sense of momentum.
In modern English, it usually describes a period when things are happening strongly, not slowly or tentatively.
- The festival is now in full swing.
- By June, wedding season is in full swing.
- When we arrived, the debate was already in full swing.
You would not usually use it for something that has barely started. If a meeting begins at 9:00, it is not in full swing at 9:01 unless the activity is already lively and fully going. The phrase works best when the action has clear momentum.