It's a word coined by pg that's sort of a term-of-art for the depressing tendency of the top HN comment to be something which pooh-poohs the company/topic/idea in the submission, doesn't add anything to the discussion, but (crucially) is worded convincingly enough to not just end up on the bottom of the thread.

e.g. "Lol that suckz because Google can steal ur passwerdz!" would not end up at the top of the thread. We wouldn't be terribly worried if someone actually said "Wait, I just audited NativeClient and it turns out you can achieve arbitrary code execution. Maybe you should avoid this software." The danger zone is comments which sound like the second but, on reflection, only tell you about as much as the first.