When you lift the top off a toilet and take a shit in it, causing a disgusting job for the person who has to clean it up. First publicly referenced in American Pie; Beta House, when the guy takes a dump in the top of the john while yelling Hocker Knocker baby!
Dude 1: So did you score with that chick who took you to her house last night?
Dude 2: No man she was a total prude and I couldn't get any
Dude 1: So what'd you do?
Dude 2: I pulled a hocker knocker and ditched that bitch!