I think that this person...meaning someone who is silly or stupid, usually for attention and in a random or sporadic way, involving nonsensical references and actions. usually violent or quick action involved, resulting in general amusement and hilarity of onlookers.
hey look, archie's jumping off that cliff while holding a banana!
wow! he's mental!
mentally-challenged; retarded; needs psychologic help
that dude's mental because he fucked his own sister.