Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Let us straitly threaten them, that they speak
            henceforth to no man in this name.    --Acts iv. 17.

   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.

            The skies look grimly And threaten present blusters.
                                                  --Shak.

   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.

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

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

Threatening \Threat"en*ing\,
   a. & n. from {Threaten}, v. -- {Threat"en*ing*ly}, adv.

   {Threatening letters} (Law), letters containing threats,
      especially those designed to extort money, or to obtain
      other property, by menaces; blackmailing letters.

Source : WordNet®

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},
             {ugly}]
     2: darkened by clouds; "a heavy sky" [syn: {heavy}, {lowering},
         {sullen}]