Work is the amount of energy transferred into or out of a system, not counting the energy transferred by heat conduction

The conduction of heat is to be distinguished from heating by friction. When a hot mass heats up your hands by conduction, the energy transfer occurs without any force, but when friction heats your car’s brake shoes, there is a force involved. The transfer of energy with and without a force are measured by completely different methods, so we wish to include heat transfer by frictional heating under the definition of work, but not heat transfer by conduction.