Ok, I'm going to give this a shot. Aptal is an adjective/noun meaning stupid, aptalca is the adverb form of this meaning stupidly. Salak is also stupid/silly/fool etc but not quite as strong as aptal. Saçma (adjective/noun) is used when someone is talking nonsense 'saçma saçma konuşma' gibi. Sersem is something like the feeling when you're not thinking so clearly similar to dazed or addled, for example when you have just woken up. Sometimes it is used as an insult to express someone is behaving in a silly fashion. Beyinsiz and kafasız are nearly the same but beyinsiz maybe slightly stronger and kafasız is often used to talk about yourself if you've done something you regret. For example kafasızlık ettim.

Thanks Simon, All the things you mentioned is right. Aptal / salak have similar meanings. In daily usuage less people know the difference. I will write the meanings I took from Turkish Online Dictionary (www.tdk.gov.tr) for the nuances. Aptal : Not very smart Salak : From his dress, his speech and his behavior, it seems that he is levelless, unstable and pure.