Somewhere between dirt poor and F*#% You Money. It's the initial indicator of modern financial freedom, allowing a person who has Tennis Shoe Money to wear trendy athletic shoes while conducting business activities that would otherwise demand a different class of foot wear. Generally centered around tech start ups, Tennis Shoe Money has spread to other business sectors.
Money saved or stored by a non-traditional method (inside of a Shoe Box) without F.D.I.C. protection, enabling easy fee-free banking from a secure private location, usually ones residence.
Penelope was perplexed at her friends concern over ATM fees or government oversight because her Shoe BoxMoney was self managed.