The act of a police officer placing their knee on the neck of a person restrained on the ground thereby cutting off oxygen. Knee-necking for prolonged periods results in the person on the ground losing consciousness and, in some cases, ceasing to breath.
Kyle: Did you see what happened to that guy in Minneapolis?
Roger: No man, what happened?
Kyle: The cops knee-necked him and he died.
Roger: Damn, that's messed up.
Tired or exhausted. Likely derived from the slang "tuckered" as in the phrase "I'm plum tuckered out", however "neckered" can stand alone without the helping words "plum" and "out".