Эй, ты че опух что ли?

18:15 Oct 17, 2019 

Thank you for always listening and being constructive, even you don't always agree!)

18:13 Oct 17, 2019 

Thanks for the insight, Katya!

18:12 Oct 17, 2019 

Also, it's super tempting to use it in the context provided by the OP (frustrated school kids insulting eachother). I couldn't resist!)

18:08 Oct 17, 2019 

Hi Boris! I know dense is associated with slow (to understand), but in modern speech, it can be used to just insult someone/show your displeasure at that person's words/actions. They don't actually have to lack intelligence for you to use it, which makes it even more annoying if it's used on you out of context. Aggressive and/or frustrated people often use it this way—they realize that what's happening is warranted, but they don't like it, and can’t think of anything else to say.

There are a lot of words that aren't quite used correctly by the general population that have either made it or almost made it, into modern US lexicon. It’s like with “prerogative”. So many people tend to use it in speech to mean that something (often despite unfavorable circumstances) is still someone’s choice: 'Well, that’s your prerogative', but prerogative actually refers to a privilege/special right, which actually makes the use of the word ironic, in such a context. People do it so often, though that I feel like dictionaries will eventually be amended to include this meaning. Basically, these kinds of faux pas sound perfectly natural to everyone but pedantic linguists.

06:37 Oct 17, 2019 

Katya, the 'dense' bit is very unlikely here. Never heard of опух being used as a stnonym for тупой, etc.

09:12 Oct 16, 2019 

We're talking about a scene where one brother (aggressor) does something unpleasant to the other brother and the victim unexpectedly retaliates, right? Is this a one-time thing? Or have tensions been building for a while and they're about to finally have it out? What have they been arguing about? Choose based on that, plus what the deeper conflict between them is centered on.
Ex. responses:
"You're the one that's lost it!" (for someone who's generally been out of line or doing all kinds of crazy things).
"You think you're all that/hot stuff" (if the aggressor has consistently been super arrogant or taking on more than he can chew).
"No, you're dense" (if there's been a lot of bad decision making in the same direction).

08:15 Oct 16, 2019 

Here's the answer: Сам ты опух!