What does 2 Samuel 5:8 mean?

On that day, David said, 'Anyone who wants to attack the Jebusites must go up through the water shaft to reach those 'lame and blind' enemies who hate David.' That's why people say, 'The blind and the lame won't enter the house.' 2 Samuel 5:8 - Modern Text Bible

(David uses the Jebusites' own words against them, and the phrase becomes a saying. The water shaft was likely a secret way into the city.)