At my shared apt. the landlady comes in the house when she wants and enters anybody's room with her key. I want to double-lock my door. How?

I saw something the other day online but then missed it, it’s something that is put inside the lock after the door is locked (it’s an old door so the key hole is large) and then basically this thing has its own key that when turned, blocks the entry of the landlady’s own key. Any ideas? Thank you.

P.S.: I mention backpacking because that was meant in the article I read diagonally