A sailor or naval officer, especially one in the British or American navy, so called from the blue jacket that was part of their uniform.
From 'blue' (the color of the jacket) + 'jacket' (the garment); this term became common during the age of sail when blue was the standard naval uniform color.
The blue jacket became so associated with sailors that entire navies were nicknamed by their sailors' uniforms—the term 'bluejacket' was so specific it became a way to show naval pride and identity.
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