Knucklehead is old-fashioned. It is also the kind of word that you might say around children - it isn't very harsh, it isn't a swear word, it is old fashioned enough that doesn't have the impact of a truly mean word. You might also use it to someone if you are not trying to be truly insulting.

Save the sermon and stick to language advice, thanks.

I do not think it is nice to call people names. Where are your manners? You probably have low self-esteem if you feel the need to insult or bully others. Maybe you think it is ok because you see it in movies or on tv, but it is not. As my late mother always said, 'if you do not have something nice to say, say nothing'. As an English teacher, I will say that it is important that you know these words and perhaps even more vulgar words, so that you know what some disgusting people may be saying to you on the street, etc.

I think the word knuckle head is a great word. I don't consider it to be old fashioned. It's a silly word so it can be used jokingly. Moron is a stronger word and is perfectly fine. That guy is such a moron! Unfortunately, in the US most of the strongest insults are now curse words. So go ahead and use dope, moron, idiot, imbecile , etc. Just be sure you are careful of the context and if you want that person to hear you. If you want a list of things to never say, or almost curse words you can message me. Or just watch US late showhosts

Knucklehead is very old fashioned, I don't hear anyone using it nowadays. Calling someone a "dope" as a noun is not common either. Moron is okay and quite common.