From the need of the boiler of a steam locomotive to exceed a minimum amount of pressure (See head (“buildup of fluid pressure”), also pressure head.) before the locomotive could start moving.
head of steam (plural heads of steam)
- (idiomatic) Amount of energy.
2011 January 25, Paul Fletcher, “Arsenal 3 - 0 Ipswich (agg 3 - 1)”, BBC:
- Arsenal were starting to work up a head of steam and Tractor Boys boss Paul Jewell cut an increasingly frustrated figure on the touchline.