Etymology is the history of the origin of words and phrases.
Not to be confused with the buggy entomology, according to the Oxford English Dictionary, the word etymology means:
1a. The process of tracing out and describing the elements of a word with their modifications of form and sense.
1b. An instance of this process; an account of the formation and radical signification of a word.
1c. The facts relating to the formation or derivation (of a word).
2. That branch of linguistic science which is concerned with determining the origin of words.
3. Gram. That part of grammar which treats of individual words, the parts of speech separately, their formation and inflexions.
English borrowed the word from Medieval French ethimologie (now etymologie in Modern French), which had in turn adopted it from Latin etymologia, who themselves pinched it from the Greek ἑτυμολογία, from ἑτυμολόγ-ος.
Etymology therefore concerns the history of words, their origins, and how their form and meaning have changed over time.
If you are asking about the history or past life of a term or phrase, as opposed to the origin of the term or phrase, please use the history tag instead.
When looking into the etymology of a word or phrase, it can often be interesting to see how multiple phrases develop over time and compared to each other. To do that, Google has an n-gram viewer, which can be used to analyse the occurrence of words in a vast number of books. Very simply put, the user inputs a number of search terms and the viewer outputs a graph an n-gram.
For more information on how to use Google's n-gram Viewer, specifically the use of wildcards and so-called inflection searches, visit the info page.