It is a very vulgar, informal, oral word, used when something seems worthy of happiness, but actually not. It's generally used for complaining or self-mocking. 'Fart' is, you know, nothing but a waste. So it generally means nothing. In most cases, '屁' is interchangeable with '屎shit', 'dick', '球ball', '毛/毛线yarn'. They all are slang. e.g. “吃饭了吗？”“吃个屁！” nothing to eat; “买个电脑吧？” “买个屁的电脑” do not buy a computer; “你赚钱了？”“赚个屁” make no money, etc.

It means "not happy at all" . It is quite rude, we use it for communicating with extremely intimate friends, just like a joke.

高兴个屁=Happy my ass

There's nothing that can make people happy hahaha

不，不是“happy fart"，happy fart译成汉语就成了”快乐的屁，高兴的屁“，和”高兴个屁“不是一回事！ 比如，你做一件让你尴尬的事，你的朋友觉得好笑，于是表现一副开心的样子，你很生气，就可以说”高兴个屁“，是在表示你对朋友的高兴表现很生气，告诉他那没有什么好高兴的。