1. One who has a declined mental state, resulting from over exposure to a cave dwelling environment and/or lifestyle.
2. An altered mental state resulting from the consumption or inhalation of bat feces.
Yeah, he's batshit crazy.
Condition may be temporary (e.g. "Dad went all batshit crazy on me when he saw the cell phone bill") or prolonged ("Adolph Hitler was a batshit crazy megalomaniac responsible for the deaths of millions in 20th-century Europe.")
see batshit, batshit fucking insane