over the transom

Refers to the idea of a writer tossing a manuscript through the open window over the door of the publisher's office. Alternatively, a "transom" is the nautical term for the back of a small boat. Something which "came in over the transom" would have suddenly (and presumably, surprisingly) entered through the "back door" of the craft, metaphorically.

over the transom

1. (idiomatic, of a work submitted for publication) Unsolicited.

   All the over-the-transom articles are handled by our interns.

   Only one piece that came in over the transom appears in this issue.

2. (idiomatic, law, government) Meeting a deadline by delivery after the day of the deadline but before opening of business the following business day.

   They worked into the night and sent an associate to make an over-the-transom filing.

• This sees adjectival use ("a submission over the transom", "over-the-transom articles") and adverbial ("came in over the transom").