When a person is shot in both knees, rendering them unable to walk.
Used to be a favourite move for the IRA, but they gave up when orthopedics became good at putting knees back together.
Oh yeah, and a kneecap is the joint between your femur and shin bones..
"Hey man, I'll kneecap 'em for you!"
1. to shoot someone in the kneecap
2. to stifle someone or something
They were arrested after they were caught attempting to kneecap a homeless man and now face a trial for assault.