Green’s Dictionary of Slang

1. (US) a scatterbrained person, usu. a woman, a fool, an idiot.

1976		in Chicago Trib. 17 Nov. A2/1: She’s no longer the erratic ‘ditz’ she used to be, and her career is blossoming, too.
1984		D. Jenkins Life Its Ownself (1985) 58: Dede Alwyn? ‘Clone.’ Sally Anthony. ‘Dits.’.
1989		C. Hiaasen Skin Tight 92: Christina had just about had it with this ditz.
1990		S. Morgan Homeboy 38: The ditz would have attracted less attention naked.
2000		M. Dibdin Thanksgiving 68: He though Lucy would be too much of a ditz to get around to making a will.
2004		L. Redhead Peepshow [ebook] Honey’s a bit of a ditz.
2005		Mad mag. May 53: [...] the loveable ditz, the oversexed nympho, the career woman struggling with motherhood.

2. (Aus.) of woman, foolishness.

2007

L. Redhead Cherry Pie [ebook] A bit of non-threatening ditz would go down well, especially as he must already have thought I was an incompetent idiot.