Judas window
 n.— «In addition to a door of communication, there was a contrivance for the effectual protection of privacy, consistiing of a sheet of glass in a hinged frame let into the wall…By means of this honestly-avowed peephole, he could at all times command a view off the outer office.…I could see through the Judas window in the wall, which I have described, but without moving from their respective places they could not see me.» —“Through a Judas Window” New York Times Aug. 23, 1874. (source: Double-Tongued Dictionary)

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