1. noun /ˈdʒæk.ɪt/
a) A piece of clothing worn on the upper body outside a shirt or blouse, often waist length to thigh length.
2. verb /ˈdʒæk.ɪt/
...to...prevent...the loss of heat...there is also a layer of silicate cotton or slag wool. This latter material is also employed to jacket the chimney for a certain portion of its length.
Wikipedia foundation.