insidious(producing serious harm in a stealthy, often gradual, manner) adjective [UK: ɪn.ˈsɪ.dɪəs] [US: ˌɪn.ˈsɪ.diəs]What an insidious assumption! = Che supposizione insidiosa.
nasty [nastier, nastiest](grave or dangerous) adjective [UK: ˈnɑː.sti] [US: ˈnæ.sti]