1 definition by Dr. Upin Disbich
When an older sibling brings a best friend into their home and the littler sibling thinks said person is their new friend and tries to impress them/flirt/or just won't leave you too alone for any reason
Your little brother has "Little Brother Syndrome" when he keeps trying to show your best friend his vast collection of Hot Wheels, even though said friend doesn't care and you both want him to go away.
by Dr. Upin Disbich July 5, 2015