The Collaborative International Dictionary of English v.0.48:

Gigantic \Gi*gan"tic\, a. [L. gigas, -antis, giant. See
   Giant.]
   1. Of extraordinary size; like a giant.
      [1913 Webster]

   2. Such as a giant might use, make, or cause; immense;
      tremendous; extraordinarly; as, gigantic deeds; gigantic
      wickedness. --Milton.
      [1913 Webster]

            When descends on the Atlantic
            The gigantic
            Strom wind of the equinox.            --Longfellow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gigantic
    adj 1: so exceedingly large or extensive as to suggest a giant
           or mammoth; "a gigantic redwood"; "gigantic
           disappointment"; "a mammoth ship"; "a mammoth
           multinational corporation" [syn: gigantic, mammoth]