"Péter is a very stupid child."
Translation:Péter egy nagyon sötét gyerek.
It is very common, but rude and definitely informal. Similes are often added eg. "sötét mint az éjszaka".
Wiktionary just lists "dark" and "somber", etc. as definitions, so I would guess it's slang. (Although I don't speak Hungarian, so you should take what I say with a grain of salt) At any rate, I'm fine as long as hülye is accepted.
"Sötét", hmmm.... maybe slang, maybe idiomatic. I don't know. But it refers to the complete darkness in one's head as a metaphor for stupidity. Or, I look someone in the eyes and all I see is darkness, emptiness, empty dark space. A blank.
A matching English term could be "dense".
It separates in usage from somebody having dark intentions and such. That is expressed somewhat differently. Maybe best decided by context.
"Ez az ember nagyon sötét" - This person is dense.
"A Sötét Nagyúr" - The Dark Lord.
"A Sötét Nagyúr nagyon sötét" - The Dark Lord is an idiot. (sorry)
Somehow it is clear which one means what.
I looked up the difference to be sure and found that idioms are more universal and longer lasting than slang expressions which change more with different groups, regions and times.
To that end I can confidently state that the dumb meaning of "sötét" has been around for decades and is known by everyone irrespective of their age, home town or social group as far as I can tell as a native, so it is probably more accurate to call it idiomatic than slang.
It is also included in a couple dictionaries I checked, even the official one. Quote:<pre>
Nagyon nehéz felfogású, gyenge értelmi képességű, buta személy, fej. Jaj, de sötét ember! Hogy lehet valakinek ilyen sötét feje! Annyira sötét szegény, hogy fogalma sincs erről a dologról.</pre>
See under I. / II. / 1. at: http://mek.oszk.hu/adatbazis/magyar-nyelv-ertelmezo-szotara/kereses.php?kereses=s%C3%B6t%C3%A9t
For beginners this dictionary is probably better: http://szotar.sztaki.hu/search?searchWord=s%C3%B6t%C3%A9t Has dense as one of the translations.
Btw congrats for your deep understanding of foreign languages, very impressive.
Other options should also be accepted, too, such as ostoba, buta, etc. It's not a slang related context here.