''Judas window''

Do you call the little peephole in the door a ‘‘Judas window’’?

Do you know why it is sometimes called a Judas window?

I’ve never heard it called that.

I poked around on google a bit, and found that they can be called a Judas Window, a Judas Hole, or a Judas Trap. The term can apply to a peephole on a front door, or can also be used for a peephole on a prison door so that the guards can look in on the prisoner without the prisoner seeing them. The peephole on some cameras is also apparently sometimes called a Judas Hole. Apparently, even two way mirrors can sometimes be called a Judas Window as they, like the peephole, allow someone to look through to the other side without being seen.

As for the origin, I found one site that said it came from an old painting where guards were watching Judas through a peephole (the painting and artist weren’t named), and I found another site that said it came from old church peepholes that priests could look out through and make sure that the person seeking entrance was not of ill intent (a “Judas”). Neither explanation had any sort of cite.

From the OED