The Oxford Dictionary defines etymology: the historically verifiable sources of the formation of a word and the development of its meaning. So? I can hear you asking.
If you use a word in your story set in Victorian London and it didn't begin to enter the mainstream until the Great Depression, you have a problem. Etymological dictionaries will save your professional credibility. For example, in my mid-grade time travel novel, I had to search out the expressions used by sailors in 1890 to make sure they were used on sailing ships. I found three instances where the words I'd used did not start until 1910 and later. I also had my protagonist use an exclamation that went out of use in the late 1700s. Ooops!
I now use two sources for help:
- Chambers Dictionary of Etymology - the print version
- The Online Etymology Dictionary at www.etymonline.com