transom

over the transom offered or sent without prior agreement; unsolicited. US informal

☞ A transom is a crossbar set above a door or window, and the word can also be used, especially in American English, as a term for a small window set above this crossbar. In former times, before the advent of air conditioning, many offices would leave these windows open for the purposes of ventilation, thereby allowing an aspiring author to take their manuscript to an editor's office and slip it through the open window to land on the floor inside. So, a manuscript that arrived over the transom was one that was unexpected. The phrase is still often used in publishing contexts, although it is no longer confined to them.

1976Piers AnthonyBut What of Earth? Editors claim to be deluged with appallingly bad material 'over the transom' from unagented writers.