To continue building a joke up until it breaks down and leaves an awkward silence. So named because it is not unlike reaching the top of a flight of stairs, then putting your foot out for another step and stumbling because there isn't one.
"Dude, she's so easy a caveman could do her."
"She's the town bike!"
"So many people have fucked her, I got HIV!"
"... You just fucking top-stepped it, man..."