Google Labs has an app that traces word usage in books scanned by google. The word “batshit” appears to have emerged in the first half of the 1960s. It climbed steadily until the 1990s and has had its ups and downs since. “Batty” has also had an upswing and is printed in books about eight times more than “Batshit”.
“Batshit crazy” emerged around 1989 or so: since 2000 its usage has only grown.
Here’s a listing of books with the term “Batshit Crazy”:
http://www.google.com/search?q=“batshit%20crazy”&tbs=bks:1,cdr:1,cd_min:1955,cd_max:1993&lr=lang_en
Happens to the best of us.