A skeleton key was a type of key that could open multiple locks.
Jean-Luc Picard regretted having used a skeleton key to unlock his mother's door, which ultimately led to her death by suicide. (PIC: "Hide and Seek")
Q later referred to him as "My old friend, forever the boy who, with an errant turn of a skeleton key, broke the universe and his own heart". (PIC: "Farewell")