Acronym for "Hooray For All Caps". Used when people talk online and decide it's necessary to type it all out in capitals. It's quite humorous, seeing as the actual acronym itself--when used as a complete sentence--consists of only capital letters. I love the irony, and so should you.
ub3rzz : CAN NE 1 TELL ME WUT HFAC MEENS?
Person : HFAC