Source : Webster's Revised Unabridged Dictionary (1913)

Alarming \A*larm"ing\, a.
   Exciting, or calculated to excite, alarm; causing
   apprehension of danger; as, an alarming crisis or report. --
   A*larm"ing*ly, adv.

Alarm \A*larm"\, v. t. [imp. & p. p. {Alarmed}; p. pr. & vb. n.
   {Alarming}.] [Alarm, n. Cf. F. alarmer.]
   1. To call to arms for defense; to give notice to (any one)
      of approaching danger; to rouse to vigilance and action;
      to put on the alert.

   2. To keep in excitement; to disturb.

   3. To surprise with apprehension of danger; to fill with
      anxiety in regard to threatening evil; to excite with
      sudden fear.

            Alarmed by rumors of military preparation.
                                                  --Macaulay.

Source : WordNet®

alarming
     adj : causing alarm or fear [ant: {unalarming}]