Expression referring to internet legend and spanishcomedian El Risitas laughing. It is mostly being used to signify hilarity, to react to 'wtf' funny stuff or to mock someone else, or ourselves.
Person 1: Guess what!
Person 2: What?
Person 3: I accidentally created a mafia...
Person 4: ISSOU
Twaimz's twin that was sent away to an insane asylum due to the weird shape of his hair. Two years later, he escaped the asylum and decided to use his head as a traintrack. His whereabouts are unknown. Please come home Issoot. The two sides of your hair just got into an argument. They'll come back together soon. #comehomeissoot
Did you hear about twaimz's missing twin?
Yea. His name is Issoot right?
Yup.
youtube.com/twaimz if you want to check it out. it's his q and a video.