Huh.
I heard that the term came from spelunkers (cavers) who often have to wade through deep piles of bat guano, and bat guano has pathogens in it that when inhaled, induces mental instability. I heard this back in the early 70's.
That story might be totally bogus, I have no phuqqin clue... all's I know is that any glimmering desire I might have had to go caving was thoroughly extinguished, and I turned my attentions to 1)riding saddle broncs, 2) rock climbing, 3)beer, and 4) p*ssy. Not necessarily in that order.
