Green’s Dictionary of Slang

(US) a water hydrant.

1950		M. Kantor Signal Thirty-Two 15: Everybody keep your eye on the johnny-pumps. Everybody carry a Stilson wrench.
1973		E. Droge Patrolman 117: Ghetto children frolicked in the spray from the ‘johnny-pumps’.
2008		McLouglin & Adcock Brooklyn Noir 3 27: In Brooklyn, to beat back the clamoring heat of summer, we open up fire hydrants — what we call johnny pumps.