While
zpengo's writeup is impressively
terse and sage-like, let me add a plainer definition:
A hissy fit is an intense emotional reaction to an offense against you. One can be said to have a hissy fit, pitch a hissy fit, or throw a hissy fit when one is in the throes of an emotional meltdown or frustrated enough to drop social pretenses and say in no uncertain terms what one wants.