Yes, it’s a real word. And it means exactly what logic would suggest it does.
I was just editing an article that mentioned a book someone published in the 1970s, and it said that the author “had hitherto been regarded as an expert”. Clearly wrong, given the tense, but I didn’t know what was right. It struck me that “thitherto” would be ideal, but surely that couldn’t be a real word! I was about to rephrase the whole thing when I decided I might as well look it up, just on the offchance.
Moral of the story: follow your instincts.
(Next week: the tragic tale of the incompetent seductress who went around sporting a go-thither look.)