“fireplug” to describe any person’s body is a new one to me. I haven’t heard that usage before.
Now for the alternate side of the question:
Is the word “hydrant” ever used in any context other than “fire hydrant”? It sounds like it ought to be synonymous with “fountain” or maybe “faucet”. A drinking hydrant? A hydrant of youth? Three Coins In The Hydrant? My kitchen hydrant is leaky.
Shall we now refer to an obnoxious male person as a hydrant?
I also tend to view fireplug as an older usage. However, Google ngrams shows a pattern that surprises me. The usage increased steadily from about 1930 to 1960, then dropped quickly and significantly in 1960. Then it started rising again and around 2000-2005 it reached the highest usage in history. That doesn’t match my experience at all.
Doesn’t match my experience either.
I grew up in NJ and it was just fire hydrant. My girlfriend who has lived her entire life in Brooklyn calls it a johnny pump.
I’ve always said hydrant but I’ve heard fire plug used.
I think fireplug was probably a New York word, that got universalized through usage in the popular media, maybe mainly comic strips.
That’s actually because Bing’s (and Google’s) algorithm automatically treats “fire plug” and “fire hydrant” as equivalents. If you click on the image links, the text in them pretty much all say “hydrant.” (I say “pretty much” because I obviously didn’t check all of them, but of the ones I did, it was “hydrant.”)
However, here’s a page about Chicago’s fire plugs and hydrants. While it doesn’t show a picture of an actual old-timey plug, it shows the hollowed-out logs that were used in the municipal water system way back in the day.
I see no reason to think that. Actual wooden plugs used to be used, as noted above, in many cities municipal water systems, before metal pipes were used. The word itself dates back to at least 1713. When modernized water distribution systems came into place and fire hydrants were installed, it seems natural to me that the old-timey word “fireplug” would retain currency (much in the way we still “dial” a phone, even though no dial is involved.) This does not seem to be something that would be specific to New York.
“Johnny pump,” on the other hand, that sounds more like local dialect.
That’s all I’ve got, though.
I install yard hydrants on occasion. They are clearly different devices than fire hydrants, they are a similar concept for different purposes.
People I knew/know in north-eastern PA are “fireplug”.
Southwestern PA seems to be hydrant with no mention of fire at all.
Lived most of my life in Michigan and it’s always been fire hydrant. The handful of times I heard the term ‘fireplug’ used it was to describe someone with a short stocky build.
Came to post the same thing. A born and bred Brooklynite, my father (b. 1920s) always called it a johnny pump. My mom, born in the Bronx, interchangeably used fireplug as well as hydrant.
There’s an old thread about this, but as kids the boys would play Johnny on the Pony (jumping over the johnny pump). Wearing skirts, the girls were usually off on the sidelines laughing as the boys would either fall over the pump or land in a pile of dog poo. Ahh, good times… 
Aside from “fire hydrant”, ranchers install these hydrants at stock tanks.
I beg to differ. I lived in Philly till past 25 and always called them fire plugs, although hydrant was not unknown to me. I used it to a taxi driver in NYC a week ago and he understood it.
I’ve lived in New Hampshire nearly 3/4ths of my life and I’ve never heard anyone say fireplug. I only know the term from books.
My guess is that it’s an old term that has long since fallen out of fashion here. For example, you’ll read about regional terms and many lists will insist that convenience stores in New England are called “spas” and soda “tonic” but neither of those things has been true for decades, and you’d be hard pressed to find someone under 80 who uses them anymore.
I used to work in a fire hydrant factory.
I could never find a place to park.
New York mayor La Guardia was often compared to a fire plug; he was a very well known figure, so use of that term to describe him might have a major reason for the distribution you see.
How funny - I just finished reading Sue Grafton’s latest Kinsey Millhone book, and in it, she used fireplug to describe one of the guys. Very timely.
To me, fireplug referring to a hydrant is like icebox for refrigerator - a generational term.
Same here. The first time I heard it, my mother-in-law said my father-in-law had calves like fireplugs.