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]