Treacherous

Part of speech: adjective (a.)

Like a traitor; involving treachery; violating allegiance or faith pledged; traitorous to the state or sovereign; perfidious in private life; betraying a trust; faithless. Loyal father of a treacherous son. Shak. The treacherous smile, a mask for secret hate. Cowper.

Synonyms: Faithless; perfidious; traitorous; false; insidious; plotting. Treach"er*ous*ly, adv. Treach"er*ous*ness, n.