Deck 7 was the seventh deck on The Doctor’s TARDIS. It had a waste tank that could be emptied by pressing a button on the TARDIS console.
Unaware of what some of the controls did on the console, The Doctor had a habit of pressing the button to evacuate the waste tank on Deck 7 when piloting the TARDIS. Upon being notified of this by River Song, the Twelfth Doctor stated that he “better avoid Deck 7”. (The Husbands of River Song)