Green’s Dictionary of Slang

eccentric, mentally unstable; thus crackiness, instability.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor IV 248/1: They do say I’m a bit cracky, but thats all my eye. I’m a drunken old b--- if you like, but nothing worser.
1869		Bucks Herald 11 Dec. 4/2: A ‘Cracky’ Premier [...] Our Premier is what men colloquially call rather ‘cracky!’ [...] The crackiness of our present Premier is of a more dangerous kind [...] upsetting everything in our constiution.
1879		Dundee Eve. Teleg. 3 Jan. 4/3: There since lived a cracky old carle in Strathmore [etc.].
1886		Cornishman 29 Apr. 5/4: Bavaria’s Cracky King. In consequence of the state of the King of Bavaia’s mind a council of regency will be appointed.
1919		(con. 1914) ‘Leda Burke’ Dope-Darling 70: I guess I’ve about two more years to run [...] Then I shall go cracky or kill myself.
1946		C.S. Montanye ‘Don’t Meddle with Murder’ in Thrilling Detective May 🌐 A gaunt-faced youth with slanted eyes, a hooked, beaky nose and a mouth half normal size. [...] I pegged the lad [...] as ‘Cracky’ Morgan.
1957		D. Niland Call Me When the Cross Turns Over (1958) 37: Why can’t I be like other women? It’s cracky, the whole damn thing.
1972		B. Rodgers Queens’ Vernacular.
1984		Partridge DSUE (8th edn) 264/2: Aus. since ca. 1925.