牙关 is rarely used on its own. It lives in 咬紧牙关 'to grit one's teeth and endure', 牙关紧闭 'jaw clenched shut' and 牙关打颤 'teeth chattering'. For the jaw as a body part, everyday speech says 下巴.
memory
关 is a pass, a gateway that can be shut. 牙关 is the gate of the teeth, and 咬紧牙关 shuts it tight.
