Definition of "grimdark"

If your character is a more-or-less ordinary little girl fighting for her life as a usual thing, that's a fairly GRIMDARK setting; it implies that there isn't any Good authority with both the power and the will to protect them.

2008 May 16, Galen, “[OT] Necro-loli”, in rec.arts.anime.misc‎ (Usenet)

Most original. Brilliantly so! What's not to like? "Jarringly non-grimdark," carpers will sneer. Oh, yeah? What's not Grimdark about – / * Droids? / * Vagina heads? / * Gundam suits (desu)?

2012 January 6, Playa, “[40k] [Tau] Roundup”, in rec.games.miniatures.warhammer‎ (Usenet)

The thought once occur[r]ed to me, that for a while they had the silver age Legion as the actual past of a grimdark iron age Legion. Which meant that in theory the grimdark adult legion could have traveled back to silver age Smallville, or the silver age Legion could have travelled back to the iron age DC "present" of the 90s.

2012 April 24, David Johnston, “Everybody Enables Superman?”, in rec.arts.sf.written‎ (Usenet)

[T]here is a more violent ‘barbarian’ tradition of fantasy – sometimes called ‘sword and sorcery’, to distinguish it from Tolkienian ‘heroic fantasy’ – […] This in turn leads through to modern ‘Grimdark’ writing, where nobody is honourable and Might is Right. George R. R. Martin’s ongoing fantasy sequence A Song of Fire and Ice[sic – meaning A Song of Ice and Fire] is surely the most successful and popular Grimdark fantasy.

2014, Adam Roberts, “What are Science Fiction and Fantasy? (And What does that Mean about How You Write Them?)”, in Get Started in: Writing Science Fiction and Fantasy (Teach Yourself), London: John Murray Learning, page 39