While adults are just starting to notice, usually with disapproval, "fail" used as a noun (as in "That's an epic fail."), my students are already racing ahead and converting it to an adjective.

We can tell that "fail" has become an adjective because it can be preceded by the quantifying adverb so, as in "I'm so fail." [Cf. I'm so happy (adj.), but *I'm so student (n.)].

As dictionaries go, you can't get much better than that towering giant of lexicography, The Oxford English Dictionary. It's always the first place serious word lovers turn when they have questions about the origins or use of a word. Yet really serious logophiles know its limitations. There are certain instances where you need to supplement the OED with a specialist work.