When your labeled by police or doctors as being an endangerment to yourself and others. Once labeled as 51/50, you get taken to a pysch ward for a 72 hour hold to see if your mentally stable.
Someone who the police, with their poor grasp of psychology, think is a danger to themselves or others. They will generally yell at someone and if they yell back enough, declare them a 51/50. FUCK THE POLICE.
I was in the hospital and some big angry police guy yelled at me and I yelled back, so they gave me the 51/50.