The neologism ginormous, meaning very large, is a humorous, emphatic fusion of gigantic (or giant) and enormous. The word first appeared in the late 1990s, and it retains its original humorous tone. It may eventually work its way into serious contexts, but that could take a while. The similar humongous, which dates from the 1960s, still has an informal ring.
Examples
For now, ginormous is almost always used in lighthearted, casual, or humorous contexts—for example:
Tim McGraw’s no match for the Country Thunder lineup, but boy can he fill a stage with his ginormous set list of No. 1 hits. [Chicago Tribune]
I’ve never heard anyone speak in ginormous, breathless sentences quite like Andy Kershaw. [Guardian]
A legendary monster said to be able to kill any creature subject to its gaze or the poison of its ginormous fangs. [National Post]
But it does sometimes appear in more or less serious writing—for example:
That’s it for the good news: the ginormous deficits the government’s run for a decade means that almost everything is financed with borrowed money. [Wired]
But they are ultra-important when it comes to explaining the machinations of ginormous institutional investors. [FT Alphaville]
The city’s public pools are free, and both Brooklyn’s ginormous Red Hook pool and the 330-foot art deco Astoria Park pool in Queens draw rave reviews. [Wall Street Journal]