by 1948, perhaps 1942, apparently originally a World War II military colloquialism, from a merger of gigantic + enormous.
Entries linking to ginormous
gigantic adj.
1610s, "pertaining to giants," from Latin gigant- stem of gigas "giant" (see giant) + -ic. Replaced earlier gigantine (c. 1600), gigantical (c. 1600), giantlike (1570s). The Latin adjective was giganteus. Of material or immaterial things, actions, etc., "of extraordinary size or proportions," by 1797.
enormous adj.
1530s, "abnormal" (usually in a bad sense), from Latin enormis "out of rule, irregular, shapeless; extraordinary, very large," from assimilated form of ex "out of" (see ex-) + norma "rule, norm" (see norm), with English -ous substituted for Latin -is. Meaning "extraordinary in size" is attested from 1540s; original sense of "outrageous" is more clearly preserved in enormity. Earlier was enormyous (mid-15c.) "exceedingly great, monstrous." Related: Enormously; enormousness.