One of my favorite writing tools is the Online Etymology Dictionary. Its primary sources include the Oxford English Dictionary, the Dictionary of American Slang, and An Etymological Dictionary of Modern English. A full list of sources is here.
Why does etymology matter? If you’re writing historical fiction, an anachronistic word can throw your reader out of the story, and it can also affect your credibility with readers and particularly with fellow historical fiction writers. I can’t tell you how many discussions I’ve heard with people venting about a writer using a term that was totally out of place for the era.
Historical accuracy isn’t the only concern, though. Some words have very unfortunate connotations or origins, and I’d rather not use words that make me look like an asshole. This concern comes up in all kinds of fiction and in non-fiction, as well.
Another great resource is the Google Ngram Viewer. It searches across all the books Google has indexed online for keywords you provide and shows you when they started to appear. For example, the term “wallflower” exists as a botanical term from the mid-1700s onward, but it wasn’t used to refer to a less-than-popular woman until 1820 and later. Although the Ngram Viewer interface is a little tricky — I’m still learning the ins and outs — it can give you a general idea of how long a word has been around and in what contexts it has been used.
For more modern works, sites like The Online Slang Dictionary are pretty helpful. I often check Urban Dictionary to get an idea of how something’s being used, but I don’t rely exclusively on anything I find there. In my experience, it can be hit or miss. At the very least, though, if I’m working on a story or a piece of software that could be referred to by an acronym, I definitely check the Urban Dictionary to make sure the acronym isn’t already being used for something totally NSFW.
A site that isn’t so much an etymology resource as it is an encyclopedia of misused terms is The Eggcorn Database, which documents phrases that are used incorrectly and sometimes in rather hilarious ways. If nothing else, it makes for entertaining reading.