The domain of the function where the rule applies is any word that has at least two letters. However, there may be some words where the function does not work correctly, such as words that start with the same letter as the last letter of the word. For example, if the input word is "banana", the output of the function would be "anab", which is not a valid word. Therefore, the domain of the function where the rule does not hold is any word that starts with the same letter as the last letter of the word.