Creates a magical barrier on a target object, such as a door, to prevent eavesdropping. The target object is said to have been Imperturbed.
References from the canon
The door to the kitchen in number twelve, Grimmauld Place had an Imperturbable Charm placed on it. This prevented the kids from using Extendable Ears to eavesdrop on the meetings there. Anything thrown toward a door which has been Imperturbed will bounce off without touching it. Ginny tested the door by lobbing Dungbombs at it (OP4).
"imperturbatus" L. undisturbed, calm (thanks to Manuel Weiss for help with this)