When something so incredible happens that you lose control and instantly jizz your pants
Person 1: "Dude when i turned that corner and saw that Taylor Swift poster i Instajizzed"
Person 2: "Yeah because she is Taylor Swift!"
When something so spectacularly amazing happens that you just can't possibly control yourself. Pure euphoria, in essence.
Waking up to that text message AND Lady Gaga was just too much. It was instajizz.