There is db file in /system/vocabulary/vocab.db
If you remove the file and restart kindle it won't help, file will be downloaded from Amazon, but if you alter db and after restart kindle it will do.
To alter db use these commands:
this removes mastered words
sqlite3 vocab.db "DELETE FROM WORDS where category=100;"
this will remove not mastered words
sqlite3 vocab.db "DELETE FROM WORDS where category=0;"
this removes word references to books
sqlite3 vocab.db "DELETE FROM LOOKUPS where id!='blabla';"
sqlite3 you can download from
https://sqlite.org/download.html
linux or mac most certainly will have it installed.