When a friend, or in some cases, somebody you don't even know hugs you at inappropriate and stupid times.
"Hmm... I can't find my pen"
*Gets hugged from his friend*
"Erm... what are you doing?"
"Just want to let you know I'm here for you in this hard time"
"Dude, get off me! You've got a bad case of Inappropriate Hug Syndrome!"