the.com/impenetrability
The smug refusal of a thing to let anything—a sword, an idea, a hint—get inside.
means The quality of being impossible to enter, pierce, pass through, or understand.
from From Latin penetrare, to enter or pass into, plus the in- of negation—literally the state of nothing getting in. Humpty Dumpty famously redefined it on a whim in Through the Looking-Glass.
Humpty's glossHe used it to mean enough talk for now.
Physics flavorTwo solid objects cannot share the same space.
Armor senseMedieval plate aspired to it, rarely achieved it.