"Bluejacket" is a term for an enlisted sailor in the U.S. Navy. In 1817, navy uniforms were regulated to require enlisted men wear blue jackets and trousers with a red vest, yellow buttons and black hat.
Although the Navy was officially commissioned in 1776, it operated without formal uniforms for over 40 years. Each ship's captain had his own regulations for appropriate attire.
Blue jackets are no longer formal uniforms for the U.S. Navy. In 1864, the blue "crackerjack," a tunic with a deep square back flap commonly associated with sailors, became the official formal naval enlisted uniform. Nevertheless, "bluejacket" has an honored place in Navy slang, and the popular "Bluejacket's Manual" is considered essential reading for new sailors.