An informal, slightly old-fashioned word for someone who has spent a lot of time in prison — not a person who went once and never again, but someone people think of as “a prison person,” in and out of jail. It is not a real bird. Long ago, English used bird for a person, especially someone in a bad spot. A “gallows bird” was someone likely to be hanged; a jailbird is built the same way, but tied to jail. You meet it in stories, songs, and newspaper colour more than in careful talk. It can sound lively or unkind, depending on who says it and how.