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Related to mask: Subnet mask

mask something out

to conceal or cover part of something from view. The trees masked the city dump out, so it could not be seen from the street. The trees masked out the dump.
See also: mask, out

somebody's mask slips

if someone's mask slips, they do something which shows people their real personality, when they have been pretending to be a different, usually nicer, type of person
Usage notes: A mask is something that covers your face to hide it.
His mask had suddenly slipped, and she saw him as the angry and cruel man that he really was.
See also: mask, slip
References in classic literature ?
Instantly a mask, wearing the well-known costume of a peasant woman, snatched his moccoletto from him without his offering any resistance.
But having been thus distinguished I could do no less than follow her with my eyes to the door where the chain of hands being broken all the masks were trying to get out at once.
They saw him raise his hand and snatch off the golden mask, and Tara of Helium in wide-eyed incredulity looked up into the face of Turan the panthan.
The mask answered, "Though you have so ingeniously discovered me, I must still speak in the same voice, lest I should be known by others.
To which the mask answered, "And are you so little versed in the sex, to imagine you can well affront a lady more than by entertaining her with your passion for another woman?
He was standing close by the lady in the mask, who was playing with pretty good luck now, and looking on much interested at the game.
There was a pause for consideration, and in the pause we put on our masks.
These masks alone will down a nigger, if we meet one.
He might have wandered on till now, but in a little room there was a young lady who had taken off her mask, on account of the place being very warm, and was sitting there alone.
If any real lynching's going to be done it will be done in the dark, Southern fashion; and when they come they'll bring their masks, and fetch a MAN along.
A freshgathered cow's tail lay on the floor, and by its side two pieces of black velvet-~y black velvet-rudely hacked into the semblance of masks.
As the light swept them, there leaped out from the walls a collection of Tibetan devil-dance masks, hanging above the fiend-embroidered draperies of those ghastly functions - horned masks, scowling masks, and masks of idiotic terror.
But I cannot tell; this same truth, is a naked, and open day-light, that doth not show the masks, and mummeries, and triumphs, of the world, half so stately and daintily as candle-lights.
Pulling off their masks and tearing their robes, they fought with one another for the nuts.
Strange as it seems in the author of these masterpieces of frank realism, Jonson at the same time was showing himself the most gifted writer of the Court masks, which now, arrived at the last period of their evolution, were reaching the extreme of spectacular elaborateness.