Wiktionary is a multilingual, web-based project to create a free content dictionary of all words in all languages. It is collaboratively edited via a wiki, and its name is a portmanteau of the words wiki and dictionary.

Like its sister project Wikipedia, Wiktionary is run by the Wikimedia Foundation, and is written collaboratively by volunteers, dubbed "Wiktionarians". Its wiki software, MediaWiki, allows almost anyone with access to the website to create and edit entries.

If a word does not exist on Wiktionary, add it. If you know nothing about etymology that's fine; your humblest contribution is still better than nothing. Someone else can add the etymology later. The minimum you need to include is the language of the word, its part of speech, and its meaning.

Wiktionary supports the standard MediaWiki API - this basically lets you return page markup in a somewhat parseable format. And some projects to make working with Wiktionary content more machine-readable: https://github.com/dkpro/dkpro-jwktl.