Green’s Dictionary of Slang

squirrel-shit n.

In phrases

nuttier than squirrel shit [i.e. extremely nutty adj.2 (2)]

extremely eccentric.

M. Harper Who Am I to Complain 22: His exploits were legendary mostly because he was ‘nuttier than squirrel shit’.
F.J. Roberts Apocalypse in Mind 9: Yeah, he was nuttier than squirrel shit. No doubt about it.