Used specifically to discuss someone's fitness level in regard to their physical health. He's panting after climbing the stairs; he's really out of shape.
Pro Tip 2/3
Exercise Importance
Highlights the effect of inactivity or absence of exercise on a person's physical condition. Ever since I stopped cycling, I feel out of shape.
Pro Tip 3/3
Not Instant
Expresses a state that occurs over a period of no exercise or activity, not immediately. I used to be an athlete, but after years of no training, I'm out of shape.