Folk etymology is when a word or phrase is changed – phonetically, orthographically, or both – to better fit a mistaken idea about its origin. It’s why some folk call a hiccup a hiccough: hic-cough may seem more plausible or comprehensible. The original impulse, says Arnold Zwicky, is “to find meaningful parts in otherwise unparsable expressions”.
So asparagus is sometimes written as sparrow grass, much as chaise longue → chaise lounge and coleslaw → coldslaw (which also count as eggcorns – sort of distributively limited folk etymologies). Many remain incorrect or restricted to small groups, but some become standard: penthouse came from pentis, lapwing from lappewinke, and hangnail is a modified Old English agnail.
Most people would probably assume that shamefaced comes from “shame-faced”, but the word was once shamefast, literally “restrained by shame” (fast as in “held firm”). The idea of shame manifesting in a person’s face motivated and sustained the alteration.
Some specialists restrict folk etymology to the process of transformation, but in lay contexts it’s also used to refer to the results, or to the false origin story itself. In any case, it’s about how we replace relatively strange elements in a phrase with more familiar ones which we then concretise in spelling or pronunciation.
Foreign words are a common source of folk etymologies. Belfry came from Old French berfrei, the ensuing ‘l’ forms being reinforced by association with bells. Spanish cucaracha led to English cockroach, while woodchuck (another name for the groundhog) probably derived from the Algonquian word ockqutchaun, according to the American Heritage Dictionary.
Acronyms real or imaginary also beget folk etymologies, or more strictly false ones, especially when their origins are obscure. Thus, posh is said to be a backronym of port outward, starboard home, but there’s no evidence for this; ditto golf from gentlemen only, ladies forbidden. SOS was chosen for its convenient communicative pattern, not as shorthand for save our souls/ship – these phrases were applied later.
This is too much fun not to join in, so I sometimes invent my own spurious etymologies. The following tweet is not so much a folk etymology as a reverse eggcorn:
Backstory: As a child I had posters covering my bedroom walls, and in cold weather a few flies would secretly snuggle in behind them and wait for warmer days, at which point they would scratchily (and yes, creepily) crawl out again like tiny creatures of the undead.
OK, so bees ≠ flies, and whether they hibernate is debatable, but this is no time for zoological pedantry. Send me your eggcorns and false and folk etymologies, actual or concocted on the spot. Let silliness be no object.