A gaolbird is a jailbird: a person who has been in prison, often more than once — spoken of as if prison were their nest. It is informal, a little old-fashioned, and not a kind name. The British spelling keeps gaol; everyone else writes jailbird. Though actually, bird here is not an animal. English has long used bird for a person (a sly bird, a jailbird), a bit like calling someone a type. In an old film it can sound jokey; about a real person it often sounds cruel, because it makes their worst days into their whole name.