The Collaborative International Dictionary of English v.0.48:

Fearsome \Fear"some\ (f[=e]r"s[u^]m) a.
   1. Frightful; causing fear. [Scotch] "This fearsome wind."
      --Sir W. Scott
      [1913 Webster]

   2. Easily frightened; timid; timorous. "A silly fearsome
      thing." --B. Taylor
      [1913 Webster]

WordNet (r) 3.0 (2006):

fearsome
    adj 1: causing fear or dread or terror; "the awful war"; "an
           awful risk"; "dire news"; "a career or vengeance so
           direful that London was shocked"; "the dread presence of
           the headmaster"; "polio is no longer the dreaded disease
           it once was"; "a dreadful storm"; "a fearful howling";
           "horrendous explosions shook the city"; "a terrible
           curse" [syn: awful, dire, direful, dread(a),
           dreaded, dreadful, fearful, fearsome,
           frightening, horrendous, horrific, terrible]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "fearsome":
   alarming, appalling, awe-inspiring, awesome, awing, bashful,
   chilling, cowardly, daunting, deterrent, deterring, diffident,
   disconcerting, discouraging, disheartening, dismaying, disquieting,
   dreadful, fear-inspiring, fearful, fearing, formidable,
   frightening, frightful, goosy, in fear, intimidating, jumpy,
   menacing, mousy, nervous, overawing, rabbity, scaring, scary,
   shaky, shivery, shrinking, shy, skittery, skittish, startling,
   startlish, terrible, terrifying, timid, timorous, trembling,
   tremulous, trepidant, trigger-happy