A portmanteau of "fan canon," aspects or material that are not officially part of the source material (of a work of fiction) but that fans promote or think could logically exist or occur in the source material. "Fanon" can be used interchangeably with "head canon," although the latter can also be applied to an individual fan's ideas, as opposed to those that have gained widespread currency. That pairing must be fanon because I definitely don't remember seeing those two kiss in any episode of the series.