stands for disappointed in you, used as a stand-in for "smh". In other words, it is a phrase used to express some form of disappointment in someone's actions. Properly used when someone messes up, lets you down, or embarrasses themselves.
"yo i just totally hooked up with this tranny last night"
"hey i can't come out tonight, i have to do homework"
-"cmon man diu"
"yo son i just applied to east virginia tech"
-"diu son that school doesnt even exist"
"hey i have a new mixtape out check it out"
-"diu man this shit sucks"