In popular usage, the term “folk etymology” is often applied to the fanciful explanations given to explain the origin of certain words and expressions.
For example, one frequently repeated “folk etymology” is that the expression rule of thumb derives from a medieval law that restricted wife beaters to a stick no bigger round than the thickness of their thumbs. It makes for a sensational story, but has no truth in it.
Professional etymologists use the term folk etymology to describe the process by which an unfamiliar word is altered through use to resemble a more familiar word.
Folk etymologies result from mishearing, mispronunciation, misunderstanding, and a desire to rationalize words that make no sense to the speaker.
Here are a few examples of words that have been altered by the process of folk etymology:
shamefaced: OE scamfaest, “restrained by shame.” The element “fast” had the sense it has in this sentence: The prisoner was made fast by chains. The OE spelling changed to shamefast, meaning “bashful,” i.e., restrained by feelings of embarrassment.” Since “fast” no longer made sense to speakers in that combination, the spelling was rationalized to shamefaced. A bashful person frequently goes red in the face.
island: In OE, the word for “island” was iegland or igand which ordinarily would have become iland in modern English. But then the word isle came into English from Old French which got it from Latin insula. The OE word can also be traced back to the language of the Romans, but the Latin word it’s related to is aqua, “water.”
kitty-corner: the expression began as “cater-corner.” Cater was an English dialect word meaning “to set or move diagonally.” Cater is itself a folk etymology of the French word quatre, “four.”
chaise lounge: The French expression chaise longue means “long chair.” To many unobservant readers the word longue computes to lounge. Hey, that makes sense. One lounges in a chair that lets you put your feet up. Now many lawn furniture departments advertise “chaise lounges.”