A commonly used word in Ireland referring mostly to teens who think they're the shit for drinking and smoking and getting up to other teenage shenanigans. Spoof of the word 'mad' meaning 'crazy'. A person who does 'shmad' stuff is called a 'shmadster'.
Boy 1: Ye goin gaff party tehnoih?
(i.e - Are you going to the house party tonight?)
Boy 2: Nahh goin field wihh dahh m8s, guna geh smashed coz we r 'shmad' loike!
(i.e - No, I am going to a field with my friends, I will be getting severely drunk because I am 'shmad' like.)
This is the reply one should use when some idiot attempts to tell you how you shoulddo something you already deal with on a regular basis and that they have never experianced and know nothing about.
Mr. Knowitall was once again getting on everyones nerves by telling them how they should complete the job when one individual shut him up by blurting out, "shaddafuckup".