It originates from
Mistel warheads used by Germans in WWII.
The definitive Mistel warhead was a shaped charge around a core of copper or aluminium. When detonated, the explosion would create a penetrator made of the core of the above mentioned copper or aluminium. The metal would then behave like a liquid (and not, as often falsely reported, liquefy), capable of penetrating up to 7 metres (24 feet) of steel armour. It was anticipated that this would be able to “drill” straight through an enemy warship.
The Mistel was never proven to be affective.