I have not bothered to look this up in any reference books, so please forgive me if the answer is blindingly obvious.
What, I would like to know, is the quality inherent in the excrement expelled by bats that is related to human madness? I am thinking of the phrase batshit crazy. We do not commonly say, for example, catshit crazy or giraffeshit crazy or puff-addershit crazy, or at least I have not heard these phrases used at the sort of swish sophisticated cocktail parties I try to barge my way into. But reference to the droppings of bats is made frequently.
Answers in the Comments please, unless you are batshit crazy yourself, in which case you would be more appropriately occupied reading something by Will Self.
Batshit is the only mammalian excrement which, from the point of view of its expeller (invariably hanging in the rafters) vanishes overhead. Hence ‘batshit crazy’ means ‘negative with respect to gravitas’.
I always thought it was a reference to Sir Henry Batshit, who was crazy.