Great Scrabble word :) According to https://en.wiktionary.org/wiki/gruntledgruntled is a humorous neologism introduced as a back-construction by P. G. Wodehouse in The Code of the Woosters (1938): 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.
