Etymology is the study of the historical origin and development of words. To take the words bag and chair as examples:
- The noun bag is probably derived from the Old Norse word baggi, and is first seen in English texts around the year 1200, when it was written bagge.
- Chair also entered the language at about the same time, but comes from the Old French word chaiere
(Source : The Online Etymology Dictionary)
Etymologists work by drawing logical conclusions about the provenance of words. These conclusions are formed by eg:
- tracing the form and use of words back in time through written texts.
- comparing the form and use of words across languages in texts written at various times, to see whether a word has been borrowed from another language (possible if the word was used in one language earlier than the other), or whether both languages took the word from a common ancestor (possible if they both introduced the word at around the same time).
Etymological explanations can often resolve some of the apparent illogicalities of a language - for example, the fact that in English a -s morpheme is used to indicate plurals with nouns, but third person singular with present verbs. Why plural in one case and singular in another?
Etymologically, however, the two morphemes are in fact quite different :
- The "s" morpheme used with nouns to indicate plural developed from the Old English plural -as.
- The "s" morpheme which is used with verbs, on the other hand, developed from the Old English 3rd person marker -aþ, which by Middle English had become -eth (hath, giveth, taketh). and only later changed to -s.