I'm guessing it is a portmanteau of 'ginormous' (itself a portmanteau that has legitimately replaced 'enormous' for my kids' generation, much to my chagrin) and

I'm guessing it is a portmanteau of 'ginormous' (itself a portmanteau that has legitimately replaced 'enormous' for my kids' generation, much to my chagrin) and 'humongous'? Maybe with some relation to 'mundo' as well.