False etymology is urban legendry disguised as the true history of a word.
A well-known bit of false etymology, for example, is about the F word. You've probably heard at some time or other that it was originally an acronym for "For Unlawful Carnal Knowledge".
Not true, but interesting, and therefore viral.
Usually false etymologies are based on the sound of a word, or the erroneous decomposition of a word that wasn't compound to begin with. An example would be where people say 'history' is a compound of 'his' and 'story' (and not, as it actually is, inherited from Greek 'historia').
Sometimes it'll even reshape the word according to its whim: You could get sparrowgrass from asparagus. For people who think 'history' has 'his' in it, nothing prevents them from creating such abominations as 'herstory'.
Learn to listen for dubious word-histories, and check them against your dictionary, which most likely will know better.