Often spelt cucul. Le Wiktionnaire gives a good definition. Sometimes written out as cucul la praline.
According to the Dictionnaire historique de la langue française cul was commonly used in the 19th century to mean someone rather uncouth and not very bright.
Cucul la praline (or cucul-la-praline) appeared in the XXth century. It's just an intensification of cucul.
Wikipedia will tell you what a praline is. Have a thorough look at what a proper praline is (not the praline belge which is just another name for Belgian chocolates) before you read on.
Still according to the Dictionnaire historique de la langue française the connection between cucul (from cul and praline) was made because of what a praline looks like (pralines can be pink as on Wikepedia's picture but the proper ones are dark brown, the colour of cooked hardened sugar).
I would say it is not considered rude to use it but it is very familiar. Although some writers have used it, I would not use it in formal speech.
Noeud can be used as a synonym of cul when referring to a stupid person. Have a look at meanings 4 & 5 in the Wiktionnaire entry.
So can con as a matter of fact, but I would say con is definitely more derogatory/insulting than the other two. Noeud and cul have a more gentle connotation, as if when using it one would pity those we are talking about.