To unexpectedly unleash deep-seated secrets that more-often-than-not delve into the TMI category and leave all those present slack-jawed.
"I don't know WHAT that was! We were just talking about crunchy peanut butter vs. creamy, and all of a sudden she opened the door in the floor and went off about a dog that died 10 years ago, and how different her life would be if she hadn't poisoned it..."
it is a euphamism for shame, guilt, maddening consequences; stemming from "A tell tale heart," by Edgar Allen Poe, where in he describes the agony of conscience. The murderer places the victim under the floorboards and drives himself crazy by imagining his heart beating through the wood.
Hey yo, every since I stole that lady's purse I've been having the door in the floor!