Old English had a set of plural pronouns that were very similar to the masculine/feminine pronouns, differing only in the vowels. The third person plural pronoun was:
Nom: hīe [hiːə], Acc: hīe, Dat: him, Gen: hira
These gradually fell out of use to be replaced by the Old Norse word þeir, originally meaning "those". This was partly because the sound changes from Old English to Middle English would have caused many of the 3rd-person pronouns to become identical. In particular, if the word hīe had not been replaced by þeir, it would eventually have been pronounced identically to "she"!
From Dr. William Long:
.9. Ditzy (1973). It is hard to believe that this word is of such recent origin. It means "eccentrically silly, giddy, or insane," according to the Collegiate. The OED calls it ditsy and points to dicty, a word first appearing in 1926, as the progenitor of ditsy. But dicty means "conceited, high-class, snobbish." Though the OED says that the origin is unknown, it seems to have emerged in Black English, as reflected in this 1944 quotation: "These (sic) are only a few dozen words and phrases that are uniquely Negro...such as 'dicty' which means trying to put on airs and act upper class without having the basis for doing so."
Thus, I have a hard time seeing how ditzy/ditsy might have been derived from dicty. In any case, once ditzy appeared on the scene in the late 1970s, it meant either "fussy"/"intricate" or, more frequently, "(Esp. of a woman) stupid, scatterbrained; cute." Even though we were in the middle of the feminist revolution at that time, the word ditzy seemed to take on a meaning associating it exclusively with women. From a Time movie review in 1981: "Bob Newhart plays the President of the United States. Madeline Kahn is his ditso wife. Gilda Radner is their ditsy daughter." Perhaps receding images of Goldie Hawn also helped to give it an anit-feminist life of its own. Or, perhaps when men felt threatened by the gradual incursions of feminist thought into the workplace, they retaliated by coining the term ditzy.
Here's the OED's 1976 earliest citation from Texas Monthly:
After a day of shopping for school clothes or a prom dress, mother used to steer us to places like Neiman-Marcus' Zodiac Room (for Pot Luck Specials), Sakowitz's Sky Terrace (shrimp salad and cheese straws), Foley's Azalea Terrace (turkey mornay), or Frost Brothers' Tastesetter (margarita pie). Besides menus of "ditzy ladies' things," these places had loads of cozy charm, an an occasional fashion show, and cheerful service.
Here's an antecedent from 1974's Myra & Gore by John Mitzel:
Here's another from a 1974 New York Theatre Critics' Reviews:
Finally, here's an antecedent for the OED's alternate spelling ditsy, from 1973's They've Shot the President's Daughter! by Edward Stewart:
I just couldn't keep my eyes off him, Fran, and I still can't believe he's interested in ditsy little me! But is he ever— oh boy! I practically had to pry him loose with a crowbar after the third "goodnight"! Anyway, wish me luck!
Best Answer
The Word Detective has this to say about it: