According to Take Our Word For It, the origins of the word disgruntled are as follows:
Usually, the prefix dis- implies a loss of something, as in disease, disgust and disgrace. Very occasionally, as here, it means “entirely” or “very”. So disgruntled means something like “extremely gruntled “. So, what does gruntle mean? It is a variant of the word grunt with the obsolete meaning of “grumble”. The word appeared sometime around 1680 but was originally a transitive verb meaning “to give [someone] extreme cause to grumble”. Incidentally, a word formed by lengthening another (like gruntle from grunt) is called a frequentative form of the earlier word.