Garibaldi's Red Shirt
The red shirt is the habitual upper garment of American sailors. Any Liverpudlian will tell you that some fifteen years ago a British tar might be discerned by his blue shirt, and a Yankee “salt” by his red. Garibaldi first adopted the American shirt, when he took the command of the merchantman in Baltimore.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894