I read in a cd booklet that Beethoven was mostly educated autodidactically and, not knowing what that meant, decided to look it up. It wasn’t in my dictionary, so I tried it online. I cannot find the definition anywhere. However, there were quite a few uses of the word on people’s web pages, such as someone learning the guitar autodidactically. From the contexts the word has been used in, I’m guessing it means something like, learned by oneself or not in an actual institution.
Is this right? And, if the word is still used, why isn’t it in dictionaries?
WhY? Because, didactic is already a word, that’s why. “Auto”, and “-ally” already have clear, independant meanings.
“Autodidact” ought to be in a reasonable large dictionary – it’s used often enough. I wouldn’t be surprised if the adjective weren’t.
And, yes. it means “self-taught”.
Well, it’s in mine- the advantages of an Unabridged.
“A person who has learned a subject without the benefit of a teacher or formal education- a self-taught person”
Pretty well straight from the Greek.
Likewise, “autodidact” is listed in the dictionary.
This actually is an issue with how words are looked up online. If you had an actual book, you’d find “autodidact” and get the rest. However, a search for “autodidactically” would bring up nothing.
Dictionaries rarely have separate entries for words with an “-ly” form; they generally list the adverb at the end of the other entry.
Finally, whoever used “autodidactically” has obviously not ever read Strunk and White. 
Perhaps the culprit was an autodidact.