Julian Burnside - A Bit About Words:
P.G. Wodehouse used gruntled as a humorous opposite of disgruntled. Gruntle came first. It means to utter little grunts. As a noun, it is the contented grunting sound made by happy pigs; it is also a pig's snout. A pig whose nose is actually or metaphorically out of joint is aptly described as disgruntled.
Cross ref: Team Random.