Is “snitty” a popular American English term? What is its origin?

I came to know the word, “snitty” for the first time from the remark of Mr. William Barr during his testimony on his way of handling of Mueller Report in the Senate Judiciary Committee. Washington Post (May 2nd) reports under the headline, ”Pelosi escalates attacks on Trump, compares him to Nixon, as Barr’s defiance angers Democrats”:

Barr dismissed Mueller’s letter as a bit “snitty” and said he believed it had probably been written by a staff member.”

Neither of online Oxford or Cambridge English Dictionary carries the word, “snitty.” But Longman Dictionary explains its meaning as “annoyed or rude to somebody because you are annoyed, together with an example, “I get snitty about bad service in restaurants.”

Google Ngram shows the word came into use around 1970, and that its currency made a rapid rise from zero up to 0.0000001853 level in 2000.

What is the exact meaning of the word, "snitty"? Is it just a slang, or "serviceable" English word as being publicly used by a man of dignity in an official situation? What is the origin of this word?

'Snitty' through the years

The earliest instance I've been able to find of snitty where the word is used in its modern sense (derived from the noun snit) is from Philip Fair, A Marriage Is Arranged: A Love Story (1932) [combined snippets]:

They looked up at Gay's step.

"'Lo, Gay, ole thing!" from Nance the younger.

Gay drew her brows together in mock astonishment.

"Nance, darling, where did you get a lingo like that?"

"Just where you got yours! It gets in your hair nowadays!"

"Don't get snitty, Gay," from Barbara, "just because you've been over talking to the handsome answer."

Gay flushed.

"Is there anything that happens on this place that you don't know of, Babbie?" she asked.

Barbara laughed.

One striking thing about the examples of snitty that turn up in newspaper articles in the Elephind database is that the word seems to be disproportionately applied to girls or women.

From Wally Trabing, "Mostly About People: (Sigh) Troy (Sigh) Dona(Sigh)hue," in the Santa Cruz [California] Sentinel (October 21, 1964), quoting a 15-year-old female Troy Donahue fan:

So, uh, we got to where they were making a scene [in the movie] and we were kinda scared to go up to him so then he was there, and we were just looking at him, and he kinda looked at us; probably thought we were a bunch of snitty girls.

From "Cybill Shepherd: From Memphis to Modeling to Movies," in the San Bernardino [California] Sun (July 16, 1972):

Cybill broke into films accidentally. Peter Bogdanovich, a young director, was strolling through a California supermarket when he spotted Cybill on the cover of Glamour. He decided to cast her in The Last Picture Show as Jacy, a spoiled, snitty, small town high school heartbreaker.

From "Walter Scott's Personality Parade," in the San Bernardino [California] Sun (May 28, 1978):

Q. Why is Princess Anne disliked so heartily by so many Englishmen?—Georgis Mason, Baltimore Md.

A. Princess Anne, 27, has been accused of arrogance, moroseness, lacking in sense of humor. She is not particularly intelligent or well-educated. She makes cutting remarks. Not too long ago, when a group of demonstrators protested her participation in a Sunday fox hunt, she testily demanded: "Who is paying you to do this?" Many of her contemporaries regard her as "snitty," resent the fact that they are taxed in order to provide her with a yearly allowance of approximately $100,000.

From a Doonesbury cartoon involving Uncle Duke, Brenner (the sometime caretaker of Duke's lodge in Colorado) and Officer O'Malley of the local police department, in the San Bernardino [California] Sun (August 22, 1981):

Duke: Look, I'm telling you it was all just a terrible accident! I thought he was a raccoon, for God's sake!

O'Malley: Maybe, but I'm going to have to take you downtown anyway.

Duke: Now see what you've done!

Brenner: You tried to kill me, man!

Duke: Look, O'Malley, let's be reasonable. Don't you have some favorite charity?

Brenner: Now he's trying to bribe you!

O'Malley: Are you trying to bribe me?

Duke: Not if you're going to get snitty about it.

Brenner: Officer, do your duty!

From Lori Mason, "James' Europeans Light and Pleasant," in the [Poughkeepsie, New York] Miscellany News (October 9, 1981):

Fine performances are also given by Karen Strain, as Charlotte Wentworth, and Anne Bergman as Lizzie Acton. Strain realistically captures Charlotte's kindness and simplicity as a character, and Bergman is effective as a somewhat snitty and jealous Lizzie.

From a column title in the Columbia [University, New York] Daily Spectator (September 16, 1991):

The Dysfunctional Box Or: Snitty 'R' Us

Dramatis Personae...

Catherine "My stomach hurts. I have to study now" Thorpe

Kirsten "I had to wait outside for TEN minutes" Fermaglich

Kristin "I don't have anything personal against BOM..." Kanthak

Liz "I can't tell you who I'm dating. It's a secret" Reza

Evan "Can we just read the damn points now?!?!" Ambinder

(Plus a Jordan, another Kirsten, a Jeremy, and a Joe.)

And from remarks by Council person Margaret Little in "Alto Council," in the [Rusk, Texas] Cherokeean/Herald (September 17, 2003):

"Sandra Wallace has dealt with me calmly and professionally and I compliment her for that. I can pay, but you (the city) are unreasonable. Gas employees have been snitty. I have encountered unprofessional behavior. I know people who don't have services from the city because of the way they were treated."

Conclusion

As these examples show, snitty isn't merely a straight synonym for being in a bad mood or being argumentative or being snappish, snippy, snooty, or snotty. Rather, it goes beyond these notions to imply a kind of childish or adolescent inability to control one's emotions and behave in an appropriately mature manner. It is thus a rather patronizing and dismissive adjective to use—especially in a professional setting—to characterize someone or (in this case) someone's letter.

from snit n. etymonline

"state of agitation, fit of temper," 1939, American English, of unknown origin. First in Claire Boothe's "Kiss the Boys Good-bye," which gives it a U.S. Southern context.

The OED registers snitty 1978, an adjective as:

slang (orig. and chiefly U.S.). Ill-tempered, sulky.

The use of the noun and adjective are infrequent in print, more frequent in spoken AmE.