by the by adjective 1prepositional phrase 1forms 2 adjective1 •Incidental; unplanned.After that, the fact of a Moussa Dembélé double cancelling out Pierre-Emerick Aubameyang’s opener seemed by the by. — 2019 prepositional phrase1 •Used to introduce a new topic; incidentally, by the way.Now, Mrs. Clarke was one of those to whom caps and crape were the very morality of mourning—she was not the only one, by the by, with whom propriety stands for principle,... — 1831 formsmore by the by · most by the by