Back in the 1600s “gruntling” meant
“grumbling.” So if someone was gruntling, they were even more
upset if they were disgruntling. The first known use of
“gruntled” as an adjective to mean “in good humor” or “pleased”
in the Oxford English Dictionary is attributed to P.G.
Wodehouse, who included this sentence in his 1938 novel The Code
of the Woosters: "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."
Instead of being
negative, the “dis-” prefix in “disgruntled” is an intensifier.
It means “utterly” or “completely” and adds emphasis to the
root.
Gruntled” is a back-formation that people derived from
“disgruntled.” In other words, so many people thought
“disgruntled” should have the corresponding positive word,
“gruntled,” that it emerged and was accepted.