Like a traitor; involving treachery; violating allegiance or faith pledged; traitorous to the state or sovereign; perfidious in private life; betraying a trust; faithless.

tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans; "Punic faith"; "the perfidious Judas"; "the fiercest and most treacherous of foes"; "treacherous intrigues"

dangerously unstable and unpredictable; "treacherous winding roads"; "an unreliable trestle"

Dangerous, not to be trusted