False etymology
A false etymology, also known as paretymology is an incorrect theory about the origin or derivation of a specific word or phrase. When a false etymology becomes a popular belief in a cultural or linguistic community, it is a folk etymology (or popular etymology). (Folk etymology may also refer to the process by which a word or phrase is changed because of a popular false etymology.)
Such etymologies often have the feel of urban legends and can be more colorful and fanciful than the typical etymologies found in dictionaries, often involving stories of unusual practices in particular subcultures (e.g. Oxford students from non-noble families being supposedly forced to write sine nobilitate by their name, soon abbreviated to s.nob., hence the word snob).