A poncho liner, also known as a woobie, is a piece of field gear originating in the United States military that can be attached to a standard-issue poncho to provide additional warmth, as well as being usable as a blanket, sleeping bag, or protective cover. It consists of quilted nylon with a polyester filling. It is attached to the poncho by means of integral lengths of material which are looped through the poncho’s eyelets.
Poncho liners were first used by the U.S. military in the Vietnam War. They gained the nickname “woobie” later; that term is conjectured to have derived from the name for a child’s security blanket in the 1983 movie Mr. Mom.
The woobies are pretty baggy, I would recommend to go for a smaller size.