knocking at your back door

a) the act of specifically raping a virgin teen girl in the anus

b) arriving somewhere unexpectedly, when they least expect you.

sociologist of criminal behavior, (visiting the cemetery of Marion State Prison, Illinois): Ghost Poinsalet, I hear you are famous, even among ghosts, in this cemetery. What have you been incarcerated, and executed for, 27 years ago?

ghost of Matthew Poinsalet (character from Dead Man Walking): Sir, it was for knocking at your back door, definition b), to the house of 2 virgin teen girls, then committing knocking at your back door, definition a), and then finally murdering my victims.

sociologist (under his breath): How atrocious, may we condemn all child rapists and murderers to the electric chair, Amen.