What does 2 Samuel 5:6 mean?

The king and his men went to Jerusalem to attack the Jebusites, who lived there. The Jebusites said to David, 'You won't get in here. Even the blind and the lame could keep you out.' They thought David couldn't take the city. 2 Samuel 5:6 - Modern Text Bible

(The Jebusites mock David, thinking their city is so secure that even their weakest could defend it.)