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.
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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.
          [1913 Webster]

                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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

threatened
    adj 1: (of flora or fauna) likely in the near future to become
           endangered; "the spotted owl is a threatened species, not
           yet an endangered one"

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "threatened":
   augured, between two fires, cornered, endangered, foreshadowed,
   foreshown, foretokened, imperiled, in a predicament, in danger,
   in desperate case, in extremis, in jeopardy, indicated,
   jeopardized, on the spot, predicted, prefigured, preindicated,
   presaged, presignified, pretypified, prognosticated, promised,
   signified