Telling someone to be quiet in a light bantering tone.
Can be used to counter someone when they've been teasing you.
Can be used in silly mucking around behaviour.
Marcy: "So Judy, are you going to wear mismatched shoes again today "
Judy "Shud it" Marcy
Person One: Sooooooo when are you going to see Markkkkky againnnn heyy...
Person Two: Shud it woman
Example Three (silly behaviour)
Person 1.... nah nah nah you suck
Person 2.... shud it