Also known as "Kneecapping". There are many references online for this term. They all refer to shooting the knee as a method of torture, or to stifle, warn or inhibit someone from doing something, without killing them. It's like a warning.

This phrase is an allusion 典故 it is like the idea of tackling someone at the knees... like many other english allusion we use an image to tell a story. imagine someone getting tackled at the knees. they would likely fall and get hurt. this is the essence of the phrase. to do irreparable damage, that starts from a weak joint. feel free to direct message me if you have other questions

It's a reference to the literal practice of hitting someone in the knees with a bat, thus incapacitating them. As far as I know, this is/was practiced by the mafia or other such hitmen, cronies, thugs. So the phrase can be used to indicate any dirty, cruel action used to eliminate an opponent, even if you don't mean it literally. "Take out someone" or "take someone out" = eliminate that person/beat that person "at the knees" = [hit them] in the knees

This can have many meanings. In business it can mean to attack the portion of the business where much of the income comes from, this harming the health of the company. In regards to a conflict between two individuals, it could mean altering something that the other person strongly relies on. Its a term that can be used in a lot of situations. I guess we really need our knees to stand and walk, if you take those out, we will fall. Then you can apply that to every day situations. Hope this helps!