What does "treacherous" mean?

Treacherous: 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. -- Treach"er*ous*ly, adv. -- Treach"er*ous*ness, n.

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