The Collaborative International Dictionary of English v.0.48:

Terrible \Ter"ri*ble\, a. [F., fr. L. terribilis, fr. terrere to
   frighten. See Terror.]
   1. Adapted or likely to excite terror, awe, or dread;
      dreadful; formidable.
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            Prudent in peace, and terrible in war. --Prior.
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            Thou shalt not be affrighted at them; for the Lord
            thy God is among you, a mighty God and terrible.
                                                  --Deut. vii.
                                                  21.
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   2. Excessive; extreme; severe. [Colloq.]
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            The terrible coldness of the season.  --Clarendon.
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   Syn: Terrific; fearful; frightful; formidable; dreadful;
        horrible; shocking; awful.
        [1913 Webster] -- Ter"ri*ble*ness, n. -- Ter"ri*bly,
        adv.
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WordNet (r) 3.0 (2006):

terribleness
    n 1: a quality of extreme unpleasantness [syn: awfulness,
         dreadfulness, horridness, terribleness]