Original: The Portuguese Man O' War (named caravela-portuguesa in Portuguese) is named for its air bladder, which looks similar to the triangular sails of the Portuguese ship (man-of-war) Caravela latina  (two- or three-masted lateen-rigged ship caravel), of the 15th and 16th centuries.
Simplified: The common name comes from a Portuguese war ship type of the 15th and 16th century, the man-of-war (named caravela-portuguesa in Portuguese, caravel).