This is the commonhappening of someone laughing loudly and exuberantly (in other words scandelously) for a LONG period of time (without the use of laughing gas) when neither the person laughing or the observer(s) know what caused the laughing in the first place.
Laughtyping when you're typing, chatting or texting someone and have that stupid look on your face like you're loving everything the other person says
Person 1: "dude who were you texting? You had a stupid look on your face"
Person 2: "my gf. I was laughtyping. We both agreed on something "
Person 1: "what's that"
Person 2: "that you're a stupid dick"