Source : Webster's Revised Unabridged Dictionary (1913)

Forbidding \For*bid"ding\, a.
   Repelling approach; repulsive; raising abhorrence, aversion,
   or dislike; disagreeable; prohibiting or interdicting; as, a
   forbidding aspect; a forbidding formality; a forbidding air.

   Syn: Disagreeable; unpleasant; displeasing; offensive;
        repulsive; odious; abhorrent. -- {For*bid"ding*ly}, adv.
        -- {For*bid"ding*ness}, n.

Source : WordNet®

forbiddingly
     adv : in an unpleasant or menacing manner; "it was forbiddingly
           dark in the corridor"