The story
Compound of moly (“molybdenum”) (informal) + cow. The term alludes to the fact that the device extracts (“milks”) technetium-99m from molybdenum-99. This same metaphor of the milk cow — a resource from which some other resource can be extracted repeatedly — also appears in the terms cash cow and lolcow.
Etymology from Wiktionary, CC BY-SA 4.0.