Originates from Invader Zim. When Gir (or anyone) has such a revelation, or stupid moment that they feel that their brain may explode from stupidity.
Simon: What is a dialog box?
Me:... are you serious? I think i'm going to suffer from head explody!
The ability to cause heads in the immediate vicinity to explode. This includes one's own head.
In Jhonen Vasquez's comic, Johnny the Homicidal Maniac, the main character 'Nny visits Heaven, having beeen denied entry into Hell. While in Heaven, 'Nny learns that, like all behind the pearly gates, the ability to explode people's heads has been conferred upon him. Very messy antics ensue, preceeded by an exclamation and a blood-thirsty grin, "I have Head Explody?!"