A knuckle-dragger is a rude name for a man you think is stupid, rough, and not interested in thinking. The picture is an ape walking with its knuckles on the ground, so the insult says this man is almost like an animal. People use it when they want to paint someone as a brute, not merely a bit slow. It is very informal, and it is meant to hurt. You hear it more in American English, and almost always about men. Do not use it about a real person unless you mean to be unkind. Though actually, people also throw it at groups they see as anti-thinking, not just at one slow man.

沒腦的大老粗（罵男性粗魯無腦，很不禮貌；帶「像猿人用指節撐地走路」的畫面）