Anything that's unconstricted isn't confined or inhibited — it's loose or free. If you prefer to be unconstricted so that you can do cartwheels and yoga poses at a moment's notice, you probably wear leggings and t-shirts rather than tight jeans and starched button-downs.
An unconstricted flock of chickens is free to roam around a yard, rather than being limited to a tiny cage. And an unconstricted party guest feels happy, relaxed, and free to dance and laugh to her heart's content. Unconstricted breathing means your breath flows freely through clear passages. Unconstricted is formed by adding un-, "not," to constricted, which comes from a combination of con-, "together," and the Latin root stringere, "to draw tight."