Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Prudent in peace, and terrible in war. --Prior.

            Thou shalt not be affrighted at them; for the Lord
            thy God is among you, a mighty God and terrible.
                                                  --Deut. vii.
                                                  21.

   2. Excessive; extreme; severe. [Colloq.]

            The terrible coldness of the season.  --Clarendon.

   Syn: Terrific; fearful; frightful; formidable; dreadful;
        horrible; shocking; awful. -- {Ter"ri*ble*ness}, n. --
        {Ter"ri*bly}, adv.

Source : WordNet®

terribly
     adv 1: used as intensifiers; "terribly interesting"; "I'm awful
            sorry" [syn: {awfully}, {awful}, {frightfully}]
     2: in a terrible manner; "she sings terribly" [syn: {atrociously},
         {awfully}, {abominably}, {abysmally}, {rottenly}]