Judas window
n.β Β«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.Β» ββThrough a Judas Window” New York Times Aug. 23, 1874. (source: Double-Tongued Dictionary)