Michael Quinion explodes etymology myths in this new series for The Telegraph. (Some are well-known but recur so often that they need to be debunked again and again, like the one about"Port Out, Starboard Home" being the origin of the word "posh". Scroll down the page for the entry.)
"On the other hand, sometimes a term really is adapted from another language or dialect, or from a once-common word that has become archaic or rare. Either way, it is strange, and so is modified into something that sounds like a current English form. Our minds search for familiar patterns and make mistakes. For example, when the second half of "bridegome" for the male half of a marrying couple became obsolete, people borrowed "groom" instead (making "bridegroom"). The Spanish cucaracha made no sense to English speakers, so they transmogrified it into "cockroach". People translate the defunct word "umbles" for the innards of a deer into "humble", as in "humble pie". British Tommies in France in the first years of the First World War heard il n'y en a plus, "there is no more", and condensed it into "napoo", finished, done for, dead; they did much the same with ça ne fait rien, it does not matter, which became "san fairy an". There are hundreds of such examples. When they first appear, they are often viewed as mistakes, as indeed they are, but they can become accepted in time."