Green’s Dictionary of Slang

1. crazy, eccentric.

1968

G. Cuomo Among Thieves 212: This one con who must’ve been seven feet tall but was kind of diddly in the brain.

2. insignificant, unimportant, worthless.

1967		Kerouac letter 21 July in Charters II (1999) 441: Any diddly piece of money will be appreciated.
2000		G.V. Higgins At End of Day (2001) 89: He’s beatin’ every one of us out of diddly spare change.