Threatening letters

from The Collaborative International Dictionary of English v.0.48

Threatening \Threat"en*ing\,
   a. & n. from {Threaten}, v. -- {Threat"en*ing*ly}, adv.
   [1913 Webster]

   {Threatening letters} (Law), letters containing threats,
      especially those designed to extort money, or to obtain
      other property, by menaces; blackmailing letters.
      [1913 Webster]