An abreviation of the popular Sci-Fi Comedy radio, book or television series or feature film Hitchhiker's Guide to the Galaxy, written by British script-writer-turned-author,Douglas Adams. Popular on internet forums as it is shorter and quicker to type than the rather ungainly HHGTTG.
Hoopy Frood A: OMG, I just love H2G2!
Hoopy Frood B: I know! I'd like to sass that Frood, Douglass Adams.
Clueless User A: WTF is H2G2?
Hoopy Frood A: Just ignore him, he has no idea where his towel is...