Brit slang for a dangerous person with a screw loose. Someone that could actually do some damage, not some harmless happy go lucky wanker.
"Fuck me rigid, Frankie Four Fingers is a right fucking Wrong 'Un. He peeled off Jimmy The Greek's fucking kneecaps with a claw hammer just 'cos he looked at 'is Old Lady funny".
A wrong 'un is a phrase used to describe someone or an action someone has undertaken that you disapprove of. It is not offensive merely an expression. Probably originates from the Midlands in England.
Billy-Bob ''I did 4 grams of coke last night!''
Jimmy-Joe ''You're a wrong 'un, you are!''