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.
      [1913 Webster]

   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.
      [1913 Webster]

            The skies look grimly
            And threaten present blusters.        --Shak.
      [1913 Webster]

   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.
          [1913 Webster]

                By turns put on the suppliant and the lord:
                Threatened this moment, and the next implored.
                                                  --Prior.
          [1913 Webster]

                Of the sharp ax
                Regardless, that o'er his devoted head
                Hangs menacing.                   --Somerville.
          [1913 Webster]

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):

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]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "threatening":
   about to be, abusive, alarming, already in sight, apocalyptic,
   approaching, at hand, bad, baleful, baneful, black, bludgeoning,
   blustering, bodeful, boding, brewing, browbeating, bulldozing,
   bullying, close, close at hand, coming, comminatory, critical,
   dangerous, dangersome, dark, denunciatory, dire, doomful, dreary,
   evil, evil-starred, explosive, fateful, fear-inspiring, foreboding,
   forthcoming, fraught with danger, future, gathering, gloomy,
   going to happen, hectoring, ill, ill-boding, ill-fated, ill-omened,
   ill-starred, immediate, imminent, impendent, impending,
   in danger imminent, in prospect, in reserve, in store,
   in the cards, in the offing, in the wind, in view, inauspicious,
   instant, intimidating, jeopardous, looming, lowering, lurking,
   menacing, minacious, minatory, near, near at hand, nearing,
   of evil portent, ominous, on the horizon, overhanging, parlous,
   periculous, perilous, portending, portentous, preparing, serious,
   sinister, somber, terroristic, terrorizing, that will be,
   threatful, to come, ugly, unfavorable, unfortunate, unlucky,
   unpromising, unpropitious, untoward, upcoming, waiting