Today I misspelled a word, went back to correct it by right-clicking and accidentally added “Septebmer” to the Dictionary; not my best moment. “Undo” wasn’t fixing the problem, so I found out how to remove that word. It’s not incredibly intuitive, but hopefully this reference can help!
You’ll need to find Custom Dictionary.txt and open it with a text editor. Save it, and you’re done!
On a Mac, you can find the file in:~/Library/Application Support/Google/Chrome
In Windows (starting with Vista), it’s in:C:\Users\%USERNAME%\AppData\Local\Google\Chrome\User Data
Update (May 5, 2016): thanks to reader, llahnoraa, for pointing out that this appears to now be located at ~/Library/Application Support/Google/Chrome/Default/Custom Dictionary.txt on a Mac.