The story
Elongated form of fu, a clipping of fuck. The specific elongation length was coined on Reddit in December 2008, in reference to rage comics on 4chan earlier in 2008. 7 "F"s and 12 "U"s are used as a reference to the /r/fffffffuuuuuuuuuuuu subreddit or its community. The term is sometimes shortened to f7u12.
Etymology from Wiktionary, CC BY-SA 4.0.