a psychological defence mechanism to deflect and repel droplets of facts that could threaten your
world view, for example
someone in a cult who refuses to listen to facts that would be difficult to reconcile with the teachings of the cult. A person deploying this device attempts to remain uninformed ("
dumb"), having deluded themselves into believing the shower of facts raining down upon them requires protection, as if sheltering under an umbrella.
When I tried to
show him one such contradiction in the cult's own
literature, he deployed his dumbrella, sticking his
fingers in his ears and walking away.