Why the free energy is called 'free'?

Free Energy refers to the energy in a system that is free to do work i.e. the internal energy minus any energy that is unavailable to perform work. Internal Energy accounts for the total energy of the system.

In 1882, the German physicist and physiologist Hermann von Helmholtz coined the
phrase ‘free energy’ for the expression E − TS, in which the change in F (or G) 
determines the amount of energy ‘free’ for work under the given conditions.
*From wikipedia*

It's normally called the Gibbs energy more recently, though at my Uni it's often been refered to as the 'Gibbs Free Energy'. I believe the same is true of the Helmhotz Energy.

Free energy is (Internal energy - energy which cannot be used to perform work) .
Free energy means that this energy is available in the form of useful work