Albert Barnes Commentary 2 Samuel 5:8

"And David said on that day, Whosoever smiteth the Jebusites, let him get up to the watercourse, and [smite] the lame and the blind, that are hated of David`s soul. Wherefore they say, There are the blind and the lame; he cannot come into the house." — 2 Samuel 5:8 (ASV)

This means, "Whoever strikes the Jebusites must reach both the lame and the blind, who are hated by David, through the water channel, and he will become chief." The only access to the citadel was through a channel worn by water (which some understand to be a subterranean passage), where, as a result, some vegetation grew in the rock.

Joab (as noted in the marginal reference) took the hint. With all the agility that had distinguished his brother Asahel (2 Samuel 2:18), he was the first to climb up. The "blind and the lame" were either literally blind and lame people, placed there in mockery by the Jebusites who considered their stronghold impenetrable, or they were the Jebusite garrison, called this in mockery by David.

Therefore, they said... That is, it became a proverb (as in 1 Samuel 19:24). The proverb seems to have arisen simply from the "blind and the lame" being hated by David, and was therefore used proverbially to refer to anyone who was hated, unwelcome, or disagreeable.