(The term was inspired by the tagline of the tabletop strategy game Warhammer 40,000: "In the grim darkness of the far future, there is only war."
Apparently GRRM is the sub-genre's generally acknowledged founder. British academic, critic, and novelist Adam Roberts describes Grimdark as a sub-genre "where nobody is honourable and Might is Right," and as "the standard way of referring to fantasies that turn their backs on the more uplifting, visions of idealized medievaliana, and instead stress how nasty, brutish, short and dark life back then 'really' was." He critically notes, however, that Grimdark has little to do with re-imagining an actual historic reality and more with conveying the sense that our own world is a "cynical, disillusioned, ultra-violent place."
An opinion with which I wholeheartedly agree.