'bluebottle' originates from English, specifically a compound of 'blue' and 'bottle', where 'blue' meant the color blue and 'bottle' referred to a bottle or a bottle-like shape (suggesting the rounded abdomen or float).

'bluebottle' was used from at least the 18th century to refer to metallic blue blowflies; later, sailors and coastal communities applied the same name to the Portuguese man-of-war because of its bluish float, and both senses have persisted into modern English.

Initially, it meant 'a blue-colored fly (blowfly)', but over time it also came to mean 'a blue, bottle-shaped floating jellyfish-like creature (Portuguese man-of-war)', broadening the reference.