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]

WordNet (r) 3.0 (2006):

threateningly
    adv 1: in a menacing manner; "the voice at the other end of the
           line dropped menacingly" [syn: menacingly,
           threateningly]