Financial professionals might recommend an "Aunt Millie" investment to clients who are unfamiliar with investing. Because the professional tries to match the investment to the customer, he or she will typically offer "Aunt Millie" a simple, low-risk investment.
Analysts may use the term to berate a stock or other security. For example, one may say that investing in a certain stock is so foolish, only Aunt Millie would buy it.