This one is listed as "rare, obs.," which as you know is not a term from the Department of Redundancy Department -- it was never very common and has since lapsed out of use except on lists of obscure or worthless words. Which is a pity, as I kinda like it. It gives the witching hour the feel of a $20 word. More or less. Either borrowed from Greek or derived from Greek roots, it's not clear which, meso-, middle + nox, night + -ian, English adjectival ending. So, literally of midnight.
I resent the implication that I am a mesonoxian troglodyte -- why, I came out of my room and saw sunlight just last week.