The story
From the resemblance of the rotable nozzle installation and its degrees of freedom of movement to that of the gun turret on a monitor (the type of warship).
Etymology from Wiktionary, CC BY-SA 4.0.
monitor nozzle
Today it means: A nozzle firmly mounted to a vehicle or structure and capable of turning completely round in a horizontal plane while having a limited play in a vertical plane, used in hydraulic mining, fire-extinguishing apparatus, etc. Full definition →