1)a: describing any noun or event as having failed; b: the state of being a failure.
2)An internet meme that has become more commonplace in "RL" amongst those of the video game/internet culture.
The use of fail and its amusement are derived from video games out of the common "game over" screens or "mission failure" notifications. Often this is used in redundancy to create a sense of irony: Fail ____ is fail: in which case any noun is thus being identified by the adjective and described by it.
-"Fail DK is fail!"