One's true personality, agenda, or motivation has been revealed, which stands in contrast to what was presented to other people. Her masked slipped for just a moment, and everyone could see the contempt she really held for her audience.

To hide, cover, or conceal something, either partially or in full. He thinks he can just mask out the smell in the bathroom with that horrible deodorant spray, but it just makes everything worse. We've planted trees around the perimeter of our property to mask it out from the neighbors.