STEAM, HEAD OF

figuratively a build-up of emotional or physical energy that one has not yet expressed or used. It often refers to intense excitement, anger or determination that drives a person into explosive action. Literally it means a supply of steam which has been accumulated in a container ready for use in driving machinery, blowing a whistle, etc. In the late 1800s and early 1900s steam was one of the major sources of power for engines of all types. Steam, under great pressure, was collected for use in the generation of mechanical power. Head here refers to the pressure of this steam and more generally to the pressure of any confined body of gas or vapor.