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Etymology is the study of the history of words and phrases, primarily reporting use, evolution, transfer between languages and such, often comprising of finding and proving (or disproving) these genetic-like relations.
There is usually overlap with morphology simply because a lot of carried meaning comes from morphemes.
The term is related to philology, which refers to the extensive study of historical linguistics, which may easily include many aspects of a language at a time - grammar, culture, relevant politics, and more.
You can usually see systematic differences between cognates between similar languages in terms of phonetics and meaning. Partly because of this, cognate guessing plays a role in second language vocabulary aquisition. Of course, it's easy to guess false cognates and false friends this way and get rather confused.
Loaning across languages can intrododuce various cognate relations.
False etymology, folk etymology
People regularly guess at etymologies. And regularly incorrectly.
They may sound quite plausible, and some get a status similar to urban legend -- they just won't die even though they're not hard to falsify.
Cognates are words that are related by origin, by common ancestry, but have started developing independently, and as such now have distinct meanings and etymologies.
Meaning may have drifted apart, or are similar and now serve to distinguish two similar concepts.
For example, the English skirt and shirt have a common Old English origin.
May be used to refer to cognates within a language, but note that words that sound about the same across languages, particularly those in the same language family, are also commonly considered cognates - often because of their common origin and their development within their language.
For example, the word 'night' looks and/or sounds similar between most Indo-European languages.
Various cognates grow from long-term development, from common origins that unites two languages in the language trees. For example, English and German are fairly closely related, while English and Spanish's common ground is mostly in Latin.
False cognates are words that are thought to be similar, usually because they look and sound it, but do not share a direct or common root.
For example, Latin habere (to have) and German haben (to have) seem like likely cognates, but these two words come from different origins. Tracing them back reveals that their similarity is coincidental; the sounds changed over time, and from distinct origins.
Doublets (etymological twins)
Repeated loaning from the same origin, often at different times, creates doublets, a.k.a. etymological twins: words with the same origin, but that are made distinct in the loaning process.
Could be considered a special case of cognates.
Regularly have similar but distinct meanings and/or uses, often (near-)synonyms, sometimes (near-)antonyms.
Examples in English: fire and pyre, aperture and overture, carton and cartoon, and, less recognizably, sovereign and soprano.
Triplets also exist, but are fairly rare.
False friends are words that look quite similar between languages, but significantly differ in meaning.
They are easily misidentified when the words are homonyms, or cognates (false or not).
False friend recognition probably happens most often across languages. Literal translation of regular sentences and especially idioms regularly introduces false friends.
- Linguistic inference / linguistic transfer