The adverb to boot is another way to say "as well" or "in addition." You could say that your cat is not only adorable, but clever to boot.
To boot is ideal for adding something extra to a statement, as it essentially means "on top of that." You might describe your best friend by saying, "She's so funny, and incredibly loyal to boot." The term comes from the Old English to bote, which was once used as part of a legal term in English law, meaning something extra that is added as part of a bargain or compensation.