This morning, I discussed an ABA Journal article, wherein Justice Scalia chastised an advocate for making up the word choate. To article reports

As Barnhouse tried to move on, Scalia offered an example. “It’s like ‘gruntled,’ ” he said.

“Exactly. ‘Disgruntled,’ ” Scalia said. Some people mistakenly assume the opposite of “disgruntled” is “gruntled,” he explained.

Well it seems Scalia may be wrong. Josh House, a GW 1L, and an astute commenter, pointed out that gruntled is in fact a word:

Interestingly enough, Scalia is wrong about “gruntled”. Oxford dictionary says it means “pleased, satisfied” – the word was derived from disgruntled in the 1930s.

Oxford English Dictionary Confirms, gruntled seems to be the opposite of disgruntled.

Definition: Pleased, satisfied, contented.

1938 WODEHOUSE Code of Woosters i. 9 He spoke with a certain what-is-it in his voice, and I could see that, if not actually disgruntled, he was far from being gruntled.

1962 C. ROHAN Delinquents 76 Come on, Brownie darling, be gruntled. 1966 New Statesman 11 Nov. 693/2 An action against a barrister for negligence..would open the door to every disgruntled client. Now gruntled clients are rare in the criminal courts. 1967 E. MCGIRR Hearse with Horses i. 17 The Agency has a nice file of gruntled exes who have found their talents in a great variety of jobs.

Gruntled is indeed in the dictionary, as the opposite of disgruntled.  But it’s pretty clearly not idiomatic, as a Google search shows.  So it’s not a mistake to assume that there is a word “gruntled” that’s the opposite of “disgruntled.”  But it is a mistake to assume that there is such a word in common usage, and especially in common serious usage (since “gruntled” as the opposite of “disgruntled” has a humorous connotation, I think).