That is an old phrase you may hear in movies. If you have a small window above a door, the horizontal bar separating the door from the window is the transom.
Here is a picture of a door with transom and an open window.
Wikipedia provides this description of the phrase “over the transom”, which is the explanation I’ve heard for years.
The phrase over the transom refers to works submitted for publication without being solicited. The image evoked is of a writer tossing a manuscript through the open window over the door of the publisher’s office
Similarly, the phrase is used to describe the means by which confidential documents, information or tips were delivered anonymously to someone who is not officially supposed to have them
You could leave the door of an office locked yet leave the transom window open for cooling. If someone wanted to deliver something to you anonymously, they could stop by your office when you are gone and throw some documents “over the transom”.
I once had a client that was in a dispute get some relevant documents over the transom. It was quite helpful.
I suppose that is how much journalism operates today.