It's some kind of slang. 열나더워 = bloody hot or Fxxxing hot or Damn it's too hot. You should'n use it actually..

Teenagers say words like 열라, 겁나, 졸라 a lot to just add emphasis. I'd say 열라 and 겁나 is just slang and they are not swear words while 졸라 is a very vulgar word. I use 졸 with my very close friends who don't judge me by how I speak. There're some other variations of this one that men use far more ofen than women, I rarely use any of them. If you have some extremely close Korean male friends, you could use it with them in my opinion.

There are no word exact “열나”, but it seems you means one of 열나다 or 열라/열라. The former means “runs a fever”. The latter is just a slang similar to “f’ing”. Examples: 1. 동생이 열나요! My brother runs a fever! 2. 열라 재밌군. That’s f’ing interesting.

Is it bad to use because it's a swear word or just because it's really slangy? Like, is it closer to "fricken" or "f*$&%in"? Or maybe kinda inbetween like the "bloody" British people use?