Because we live in a world where it's easy to block offensive words, so much so that the powers that be like to pretend that blocking talk about the 'cest is somehow an effective tool in combating it. (When instead it just coins an endless stream of new words that act as synonyms for 'the bad words'. 'Cause funk you. Funk you to heck!)

Originally it was from the supernatural community writing shipfics about Sam and Dean Winchester. Eventually the meaning diluted, and you had 4chan spouting "incest is wincest." Reddit's core userbase was poached from other web 2.0 sites, including 4chan, and so the echoes of imageboard culture live on.

Most people: "Why incest!?"
Reddit: "Why not incest?"
Reditors: "Wincest!!"

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