Abbrerviation for "Your Kink Is Not OK" -- a bad and somewhat hypocritical thing to say within the BDSM community, where everyone has some kind of kink by vanilla standards. Originated on the Usenet newsgroup alt.sex.bondage as a description of a bad attitude. Accusing someone of YKINOKism is saying that they're refusing to accept that an activity that doesn't work for them might still work well for others -- that kind of intolerance inhibits the open discussions that the newsgroups exist for.