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:

Threaten \Threat"en\, v. i.
   To use threats, or menaces; also, to have a threatening
   appearance.
   [1913 Webster]

         Though the seas threaten, they are merciful. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

threaten
    v 1: pose a threat to; present a danger to; "The pollution is
         endangering the crops" [syn: endanger, jeopardize,
         jeopardise, menace, threaten, 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"

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "threaten":
   abuse, admonish, advise, afflict, aggrieve, alert, anticipate,
   apprehend, approach, augur, await, be destined, be fated,
   be imminent, be in store, be to be, be to come, befoul, bewitch,
   blight, bludgeon, bluster, bluster out of, bode, brew, browbeat,
   bulldoze, bully, caution, come, come on, comminate, condemn,
   confront, corrupt, cow, croak, crucify, cry havoc, curse, damage,
   daunt, defile, demoralize, denounce, deprave, despoil, destroy,
   disadvantage, disserve, distress, do a mischief, do evil, do ill,
   do wrong, do wrong by, doom, dragoon, draw near, draw nigh,
   draw on, endanger, envenom, expect, face, forebode, foresee,
   foretell, forewarn, forthcome, gather, get into trouble,
   give fair warning, give notice, give warning, hang over, harass,
   harm, have a premonition, have a presentiment, hector, hex, hope,
   hover, huff, hurt, impair, impend, imperil, infect, injure,
   intimidate, issue an ultimatum, jeopardize, jinx, lie ahead,
   lie over, look black, look for, look forward to, look threatening,
   loom, lower, maltreat, menace, mistreat, molest, near, notify,
   outrage, overhang, persecute, plan, play havoc with, play hob with,
   plot, poison, pollute, portend, preapprehend, predict, prejudice,
   presage, project, prophesy, put in jeopardy, savage, scathe,
   sound the alarm, systematically terrorize, taint, terrorize, tip,
   tip off, torment, torture, utter a caveat, utter threats against,
   violate, warn, warn against, wound, wreak havoc on, wrong