Judas window
n.— «Classroom surveillance is nothing new. Most classroom doors have a glass section—a Judas window in chalkie parlance—allowing management to surreptitiously spy on Sir’s lesson.» —“Yes, head boys must have the latest toys” by Hugh Reilly Scotsman (Glasgow, Scotland) Mar. 9, 2005. (source: Double-Tongued Dictionary)
Further reading
What’s up with ketchup? The shifting history and spelling of the name of this condiment. For centuries, ketchup was a fishy sauce originating in Asia—the tomatoes came only later! Plus, a new book shows what’s lost when a language dies: when there...
The Merriam-Webster.com dictionary now includes an entry for TL;DR, also rendered as TLDR. It’s a speedy way of dismissing overlong text or introducing a shortened version of something. This useful locution also inspired AI;DR, short for AI;didn’t...