1540s, "condition of being a lunatic," formed in English from lunatic (q.v.). Originally in ref. to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The O.E. equivalent was monaðseocnes "month-sickness."
On Thursday I was diagnosed with a more specific form of mental illness than that of depression and anxiety. I have spent all weekend dealing with it, second guessing every action and reaction, and trying to determine what really makes me who I am. The best way (of course) to begin the process of understanding this news is to write. I thought it would be interesting to write a piece where the protagonist thinks she is stalked by the 'black dog', but finds that she is, in fact, the very object of her fear. I am going to link this piece to the story of Little Red Riding Hood, swapping the wolf for the black dog (although they are most likely one and the same).
You may be interested to know that Thursday night was a full moon here in Australia, and when I realised that I had a wry laugh to myself; oh the bitter irony of it all.