“Aunt Millie” (often said to be from Omaha or Dubuque) is Wall Street’s term for the average, unsophisticated investor. “Aunt Agatha” is another name that is sometimes used.
 
Personifying someone with an “aunt” or an “uncle” name is commonplace and cannot be traced to a specific date; “Aunt Millie” is cited in a Wall Street glossary by at least 1991.
 
An earlier (1954) and more established name for a small and unsophisticated investor is “Aunt Jane.”
 
 
Investopedia
Aunt Millie
A slang term for an uneducated or unsophisticated investor. The term is considered a derogatory remark in the financial sector, often used to refer to poor investment choices.
 
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. 
 
Answers.com
Financial & Investment Dictionary:
Aunt Millie
Derogatory term for an unsophisticated investor. Wall Street professionals may say that “This investment will interest Aunt Millie,” meaning that it is simple to understand. It may also imply that such small investors will not be able to appreciate the amount of risk posed by the investment relative to the opportunity for profit. Brokers and financial advisors, using the Know Your Customer rule, should not recommend complex and risky investments to Aunt Millie investors.
   
Google Books
The Complete Words of Wall Street:
The Professional’s Guide to Investment Literacy
By Allan H. Pessin and Joseph A. Ross
Published by Business One Irwin
1991
Pg, 38
AUNT AGATHA See AUNT MILLIE.
AUNT MILLIE This term is representative of the small shareholder; that is, the unsophisticated retail investor who relies on ...
 
8 November 1992, Boston (MA) Globe, “Institutional Investing, Computer Trading Leave the Boston Stock Exchange Fighting for its Life” by Kimberly Blanton:
The auction exchanges are a place where “Aunt Millie from Omaha” can trade on the same floor…
   
Google Books
Dictionary of International Investment and Finance Terms
By John Clark
Published by Lessons Professional Publishing
2001
Pg. 21:
Aunt Millie A colloquial US term for an investor who lacks experience and sophistication.
   
Google Books
The Complete Investment and Finance Dictionary:
The Most Thorough and Updated Reference Available
By Howard Bryan Bonham
Published by Adams Media
2001
Pg. 43:
“Aunt Millie.” A slightly derisive phrase referring to a public buyer of stock who owns only a few stock shares and does not trade regularly, as compared to more sophisticated institutional investors, brokerage professionals, active professionals, and traders.
   
New York (NY) Times
Tales From the Nasdaq Casino: The Winners Get Stiffed
By FLOYD NORRIS
Published: March 2, 2001
(...)
Speaking privately, some Wall Streeters express scorn for day traders. ‘‘They are just momentum players who don’t care about fundamentals,’’ one said. ‘‘This is not your Aunt Millie from Dubuque,’’ another said.
     
Google Books
Barron’s Finance & Investment Handbook
By John Downes and Jordan Elliot Goodman
Published by Barron’s Educational Series
2003
Pg. 205:
AUNT MILLIE derogatory term for an unsophisticated investor. Wall Street professionals may say that “This investment will interest Aunt Millie,” meaning that it is simple to understand. It may also imply that such small investors will not be able to appreciate the amount of risk posed by the investment relative to the opportunity for profit. Brokers and financial advisors, using the KNOW YOUR CUSTOMER rule, should not recommend complex and risky investments to Aunt Millie investors.
   
3 October 2005, Pensions & Investments, “NYSE board’s boss” by Gregory Crawford:
But Aunt Millie in Omaha, who calls her broker, it could be that she wants the ...
   
Forbes.com
The Forgotten Investor
Laszlo Birinyi Jr. 06.05.08, 6:00 PM ET
Forbes Magazine dated June 30, 2008
(...)
One remembers when electronic trading was going to lower trading costs and level the playing field. Sure, but level it for whom? Am I supposed to believe that the brokerage community is saving seven milliseconds on behalf of my Aunt Millie?