Clearly the meaning has nothing to do with "parsimonious", and it should be pronounced with a hard "g".What brings these unusual and stingy visitors to our shores is not a mystery. Blame it on the weather.
A few online dictionaries (e.g., Wiktionary) recognize both the meaning and the pronunciation, but none of the majors. Hence one assumes this usage is fairly recent.
A description of the man'o'war brings forth some other interesting terms: siphonophore and zooid. Here's the source in case anyone's interested:
https://chesapeakebaymagazine.com/painf ... a-beaches/