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Etymology is the study of word and phrase origins. Properly done, it is a critical component to understanding the evolution of, and relationships between, languages. Bad etymology, on the other hand, is often used by political agitators as a propaganda tool, often with the intent of smearing the subject denoted by the word or phrase in question or illicitly claiming credit for something the false etymologist has no right to.
Etymology comprises a significant component in several major dictionaries, including, in English, the Oxford English Dictionary and the American Heritage Dictionary. (If lexicographers devote lots of space to it, that makes it Really Important.) Etymology also provides the subject matter for countless books of language trivia and wordplay. Word etymologies can serve in historical linguistics to (very broadly) track migrations of the speakers of language groups based on changes in word meanings, as well as to (very roughly) date branches in a language's family tree. A smattering of etymological lore can help in language learning and provide psychological insights into different metaphor-systems and modes of thought.
Misuse of etymology is often called the "etymological fallacy" -- confusing the current meaning for its origins.
Word origins can take on a more sinister aspect, however, when people create incorrect etymologies for certain terms (a notorious example in US culture being the incorrect claim about the phrase "rule of thumb"); such false etymologies generally serve a political or religious purpose and are generally rejected by mainstream scholarship, but due to their propagandistic and often populist nature (sometimes augmented with a conspiracy theory or two) often find a tenacious foothold in some sections of popular culture.
Etymologies are also often affected by prejudices and superstition, a very common form of this being "taboo deformation", where the use of a word outside certain contexts is considered either offensive or disrespectful, and the actual word is replaced with either an intentional mispronunciation or a euphemism. Taboo deformation often exists within a primarily religious context, such as the conversion of the ancient Hebrew deity name YHWH (probably pronounced as Yahweh or something fairly close) to Jehovah, or the hypothesized distortion of the Proto-Indo-European words for "wolf" (wlkwos) and "bear" (rktos) into mispronunciations such as the Latin lupus or euphemisms such as the Russian medved ("honey-eater").
 False etymologyies
On the lighter side, there are countless "false etymologies" current in the popular imagination, often for words whose derivation is not certain. The origin of words such as okay and the epithet bloody are two examples.
 Unexpected etymologies
Many etymologies are striking because they show such great changes from the original meaning. Here are some examples to illustrate the fallacy of assuming that the "original" meaning is somehow the "true" or a privileged meaning:
- average began as "a duty, tax, or impost charged upon goods", then "any charge or expense over and above the freight incurred", "the expense or loss to owners, arising from damage at sea", "the distribution of the aggregate inequalities" (this is still a term of art in nautical law, where "general average" is a way of sharing the losses among all of the merchants with goods on a damaged ship), and finally, "the arithmetical mean so obtained".
- explode began as "express displeasure at a performer by clapping or hissing, to force the performer off stage".
- foreign, forest, forfeit (as well as the French hors and faubourg) all come from the Latin "foris", meaning "out of doors".
- leave began in English as "remain" (as can be seen in its German cognate, bleiben). It originates from a root meaning "sticky".
- madrigal, matrix, matter, material all have their origins in the Latin word "mater" ("mother').
- robe began as "something stolen" (as can be seen in its relative, "rob").