Anti Aim is the quality Life! Enjoy Missing Any Hit in Fake Hitboxes

Anti-aim flips the hitboxes. Your head is at your feet and your feet are at your head. If a bullet hits you in the feet, you'll be hit in the head. This basically means that any good players (and aimbotters) will aim for your head and only do foot damage.
more examples
Anti-aim, or "aa" is a type of hack mostly used in source games. Its purpose is to make aimbots harder to headshot you