Dangerous, or to be in a risky situation that should be avoided.
Guy 2- Naw, thats fuckin' shanterous
*Guy 1, 3 and 4 all nod in agreement*
Guy 1- "Look at that cunt over there! Is he eating a brick?"
Guy 2- "aye, I heard he once Killed a man with his own face, best to stay away from him, he's a shanterous one".
Guy 1- "aye shanterous indeed"
Passing English chap- "Oh I say what a shanterous looking rougue!