Bip39 Passphrase. Then inside a theft or threatening scenario, you may give your attackers the recovery seed phrase (without the passphrase) and they might recover the wallet with out figuring out you've another wallet using a passphrase For words that have only 3 letters, there is not any 4th letter. https://gen-directory.com/listings13144762/bip39-python-https-linktr-ee-bip39-bip39-mnemonic-generator-an-overview