Where do I use the weapons key?

You use this in the secret room to unlock a box which contains tranquilizer darts and a crossbow. You can shoot granny with it and she will be unconscious for a while depending on your level of difficulty you are playing.

You can use weapon key to get crossbow and tranquilizer darts on the second floor third room after pushing the boxes and going down the way, where you get a locked box.