Mid-20th century. Blend of gigantic + enormous
http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=561509261
I find "ginormous" (apparently pronounced "jye-NOR-mus") mildly annoying, but I've learned that getting cranked up over such inventions is a losing game, as a glance back at the history of attempts to stem the tide of change in English illustrates. Jonathan Swift fought strenuously against such new "monstrosities" of his day (1710) as "mob" and "banter," as well as (believe it or not) the contractions "I'd" and "can't." Benjamin Franklin violently objected to such newly coined (as of 1789) verbs as "notice" and "progress." And even in the 20th century, the revered grammarian H.W. Fowler threw a major hissy fit whenever he came across any word that combined Latin and Greek roots, including among his targets such scandalous mongrels as "bureaucrat," "electrocution" and "coastal." None of this apoplexy made a bit of difference, nor is it likely to in the future. English has always been a democracy where we vote with our mouths, and if "ginormous" doesn't prove useful over time, it won't last.
As to why "ginormous" is so widespread at the moment, I think we can blame the popular 2003 film "Elf," in which Will Farrell's character "Buddy" used the word repeatedly. But "ginormous" has actually been around for a remarkably long time. According to the etymologist Eric Partridge, it first appeared in Britain as Royal Air Force and Royal Navy slang during World War II, thereafter spreading into general population as youth slang.
http://www.word-detective.com/061405.html#ginormous