@Mrs_Who – P.G. Wodehouse gave rise to that meaning of the word in 1926. Before then it meant to grumble, or if you prefer:
“The first sense of gruntle was of a repeated grunt, especially the noise that pigs make in company. An example is in The Life and Death of Mr Badman, by John Bunyan, of 1680: “After this his speech went quite away, and he could speak no more than a Swine or a Bear. Therefore, like one of them, he would gruntle and make an ugly noise, according as he was offended, or pleased, or would have any thing done.” It is rarely used of humans, but an example occurs in a 1922 book, The Covered Wagon, by Emerson Hough, “They dismounted. The two Indians, short, deep-chested, bow-legged men, went to the packs. They gruntled as they unloaded the two larger mules.” Gruntle appeared in the fifteenth century; by the end of the next century it had begun to be used to mean grumbling or complaining. I imagine it as old-retainer mumble, the noise that someone fed up with their condition will make under their breath all the time.”
http://www.worldwidewords.org/qa/qa-dis1.htm