To make yourself seem important without actually doing anything to benefit society.
Typical USI sufferer-
"Hey guys!! i just posted an awesome new drawing on DA, and then i posted a vlog on YouTube about what happened to me today! i also talk about my new band! i'm putting some vids on my channel cuz i know we're gonna make it big! i also just put up some new pictures of me on my Myspace (lol i dyed my hair, i look so HAWT!!!) comments PLZ THX GUYS!!! <3"
Unidentified Sex Injury
Those bruises, cuts, muscle strains, and torn ligaments that happen after a night of vigorous sex - and you cannot remember exactly what you did to cause it.
"Holy crap! My one leg is completely covered in bruises, but I don't remember what we were doing and how it got that way!"
"Dude, that is a serious USI."