Douglas Harper's Etymology Dictionary
Usage examples of "peep-hole".
It was no cassock-clad verger that entered, however, but two young people, far too much interested in each other to gaze upwards towards the frets of the peep-hole.
And in some parts against Brassing, by what I've heard say, the folks fell on 'em when they were spying, and broke their peep-holes as they carry, and drove 'em away, so as they knew better than come again.
A little peep-hole enabled him to see the patient and the X-ray apparatus, while an arrangement of mirrors and a fluorescent screen enabled him to see exactly what the X-rays were disclosing, without his leaving the lead-lined cabinet.
The warden's office was in the prison building, and in order to reach that official from the prison yard one had to pass a gate of solid steel with only a peep-hole in it.