Flounce is a term used on Internet chat forums to describe a type of post. Typically a flounce is where a user feels that they have had enough of the forum and, rather than simply leaving quietly, decide to post a long public message detailing their reasons, usually in a new thread so as to get the maximum number of replies.
The flounce post could contain insults directed at fellow members of the forum or at its administrators, and the users' grievances with them. They can range from the calm and reasoned to the emotional and hysterical, garnering comment and concern respectively. After flouncing, the user, rather than staying away, is usually back within a few days to a few weeks, often expecting that the mood regarding them will be exactly the same as it was when they left - which of course it never is.
In some cases, the flouncer spams the forum or - if in a position to do so - closes it entirely or disables a certain feature of it. The word flounce is intended to evoke the image of a child having a tantrum and flapping its arms about while screaming for attention.