It's been our teenager's adjective of choice for things she's really pleased about for a while. For example, most of her Christmas presents were declared epic (to our great delight). I don't know exactly how widespread this weakened teenage use is, but my students don't tend to use it in this way. That could be due to age (my daughter's 13, my students 16-19) or geography (we're in Leicester, I teach in Nuneaton). Students in the college I work in are aware of it, though. GCSE students working on writing film reviews criticised a sample student review of Pirates of the Caribbean because it used the word epic, which they saw as slang usage and therefore inappropriate.
Is this a sign that some of these words which are used differently by teens could gradually lose their original meaning, as people no longer are aware of them? It hasn't happened to some of the reversed meaning teen slang words like 'sick'; the earlier meanings still stand alongside the new ones, but I'm pretty sure many teens don't know any other meaning for 'blatantly' than 'clearly'.
Is epic a teen word for good/great where you are?