The Collaborative International Dictionary of English v.0.48:

Threaten \Threat"en\, v. t. [imp. & p. p. Threatened; p. pr. &
   vb. n. Threatening.] [OE. [thorn]retenen. See Threat, v.
   t.]
   1. To utter threats against; to menace; to inspire with
      apprehension; to alarm, or attempt to alarm, as with the
      promise of something evil or disagreeable; to warn.
      [1913 Webster]

            Let us straitly threaten them, that they speak
            henceforth to no man in this name.    --Acts iv. 17.
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   2. To exhibit the appearance of (something evil or
      unpleasant) as approaching; to indicate as impending; to
      announce the conditional infliction of; as, to threaten
      war; to threaten death. --Milton.
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            The skies look grimly
            And threaten present blusters.        --Shak.
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   Syn: To menace.

   Usage: Threaten, Menace. Threaten is Anglo-Saxon, and
          menace is Latin. As often happens, the former is the
          more familiar term; the latter is more employed in
          formal style. We are threatened with a drought; the
          country is menaced with war.
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                By turns put on the suppliant and the lord:
                Threatened this moment, and the next implored.
                                                  --Prior.
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                Of the sharp ax
                Regardless, that o'er his devoted head
                Hangs menacing.                   --Somerville.
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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.
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WordNet (r) 3.0 (2006):

threatening
    adj 1: threatening or foreshadowing evil or tragic developments;
           "a baleful look"; "forbidding thunderclouds"; "his tone
           became menacing"; "ominous rumblings of discontent";
           "sinister storm clouds"; "a sinister smile"; "his
           threatening behavior"; "ugly black clouds"; "the
           situation became ugly" [syn: baleful, forbidding,
           menacing, minacious, minatory, ominous,
           sinister, threatening]
    2: darkened by clouds; "a heavy sky" [syn: heavy, lowering,
       sullen, threatening]