UDI'S are basically referred to as Unidentified Drinking Injuries. These occur when you are drinking of course. New injuries from drinking aren't noticed till after you are sober, you can not for the life of you remember how you received said injury, but it's obviously due to too much alcohol consumption from the night previous. When drinking you easily obtain bruises as well, so if you were drinking last night and you wake up with hella new bruises, you know why.
Friend one: "Damn dude! I woke up this morning with a fuckin' golf ball sized bruise on my thigh, what happened?!"
Friend two: "How many other UDI's
did you get from last night??? I think I saw you get that one, you knocked right into a table!"
Hungover friend one: "I have nine new bruises from last night..."
Hungover friend two: "UDI's?"
Hungover friend one: "fuck, yup."