Short for, "You mad stupid son", an alternative way of letting someone know that they're a complete a dumbass. Popular when texting or chatting on an instant messenger.
(Completely dark outside)
Yarid: Yo, what are all of you doing up drinking? Is it morning already? Did I wake up too late?
Jat: Yo, it's only 1:30am, YMSS.
Yarid: Yo, what are all of you doing up drinking? Is it morning already? Did I wake up too late?
Jat: Yo, it's only 1:30am, YMSS.
by JAT-APD August 25, 2010