Straight from the TLF

II.− Adj. inv., fam. Niais, un peu ridicule. Être cucu. Le massif de silènes et de myosotis faisait cucu (Colette, Chatte,1933, p. 70).^TLF

So vacuous, silly. The wiktionary gives corny^Wikt.

This word seems to have a lot of variants, which is not abnormal for a familiar word. I have found so far

• cul-cul
• cucul
• cucu

It is familiar, though I for one don’t consider it slang. It sounds somewhat old-fashioned to me, and actually a bit cucu itself.

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.

With all respect to the Wikitionnary answer, I feel they do not clearly indicate the true meaning this common word is supposed to imply. In Québec, it has become very common expression nowadays to indicate something as being ‘corny’, or maybe even better, ‘cheesy’.

try the website http://www.expressio.fr: it’s a gold mine for French expressions. Cucul la praline means someone a bit simple and ridiculous. For my part, I also understand it as someone a bit naive and old-fashioned. Here the link thereto:
http://www.expressio.fr/edj.php?id=CSNR935R&exp=EACY647J
enjoy it!