A double door where only 1 door has two handles on each side, and a door with no handles. Commonly, the inactive door (no handles) has something on the side of the door to open that door.
You: "Why doesn't this door open?"
Friend: "That's a Frenchdoor. Open the other one."
An acrobatic sexual act, achieved by a gentleman inserting his testicles and scrotum into a lady's vagina and having her simultaneously perform oral sex on him.
"...but Sarah, she was dirty as hell. Gave me a FrenchDoorknob on our first night together. Anyway, will the jury please deliver its verdict?"