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.

Threaten \Threat"en\, v. i.
   To use threats, or menaces; also, to have a threatening
   appearance.

         Though the seas threaten, they are merciful. --Shak.

Source : WordNet®

threaten
     v 1: pose a threat to; present a danger to; "The pollution is
          endangering the crops" [syn: {endanger}, {jeopardize}, {jeopardise},
           {menace}, {imperil}, {peril}]
     2: to utter intentions of injury or punishment against:"He
        threatened me when I tried to call the police"
     3: to be a menacing indication of something:"The clouds
        threaten rain"; "Danger threatens"