One of my favourite words is "Disgruntled" While it may be difficult to know what exactly that means, I do feel confident that I am that thing most of the time.
The prefix dis usually means opposite or not. But to assume that disgruntled means not gruntled is incorrect. Another use of the prefix is as an intensifier. In fact it is the archaic use of the prefix dis that was applied to the term gruntle.
So we are now dealing with feeling more gruntled as opposed to not gruntled.
So what does gruntle mean? Well it means to grunt or grumble indicating disagreement (note the different use of the prefix dis) dissatisfaction, or unhappiness (Dishappiness? nah).
So when I am disgruntled, I am more unhappy. So if I can cure myself of my disgruntlement, I will be back to my normal level of unhappiness.
The opposite of disgruntlement is not being happy, but being less unhappiness.
That's weird.