Locating the file in which a dictionary word is stored
Through a process of elimination, I have determined that the word is (erroneously) in the system wide spell checker.

The file isn't in your local spell check dictionary.  This is easy to check - simply edit the file ~/Library/Spelling/LocalDictionary.  You can also issue the following command to search directly from the command line
% grep -i anbalagous &lt; ~/Library/Spelling/LocalDictionary





Apple utilizes a spell checker called Hunspell which is an open source spell checker based on MySpell. (also found on https://opensource.apple.com)  The dictionary (en_US.dic) are plain text files and can easily be viewed/edited.  Issuing the following command on the downloaded dictionary file yields no results; it's not in the public dictionary from Hunspell.
% grep anbalagous &lt; hunspell-en_US-2019.10.06/en_US.dic





AppleSpell appears to be their proprietary spelling and grammar checker product as it isn't found on https://opensource.apple.com/source/ (listing of open source projects used by Apple).  The version.plist file found in /System/Library/Services/AppleSpell.service/Contents makes reference to the &quot;ProjectName&quot; being &quot;ProofReader&quot;.

 

There are additional word lists (spelling dictionaries) found in /usr/share/dict.  Searching each one for the word in question produced no result either. Eg:
% grep -i anbalagous &lt; ~/usr/share/dict/words





On a FreeBSD 12-1 VM, I installed the hunspell and en-hunspell (en-US dictionary) ports.  Anbalagous is identified as misspelled; so the hunspell engine and dictionary appear to be working.



Finally, on a clean install of Catalina, I tested the word anbalagous and it doesn't trigger the spell check.  It appears it's in the spell check dictionary/word list supplied from Apple or there is possibly a bug in Apple's implementation of the spell checker.

Since I can't seem to find a dictionary with that misspelling and the open source tools which Apple bases their spell checker seem to function correctly, I filed a bug report with Apple at https://www.apple.com/feedback/macos.html; I encourage you to do the same.