1. If someone or something builds up a head of steam, they get enough energy or support to make something happen. Agitation for reform had built up a head of steam sufficient to make it inevitable. Note: Verbs such as work up, produce or gather can be used instead of build up. The campaign is gathering a head of steam. Note: You can also use a head of steam to talk about a situation where there is a lot of support for something. While most MPs still believe an election next year is more likely, there's an increasing head of steam behind November. They need to get a sufficient head of steam to force the Foreign Office to act on their behalf.

2. If someone builds up a head of steam, they gradually become more and more angry, anxious, or emotional about something until they can no longer hide their feelings. She had built up a head of steam while waiting to speak to him. Note: Verbs such as work up or get up can be used instead of build up. Now well into his mid-30s, Elton still manages to work up a head of steam over little things. Note: A steam engine can only work when the steam has reached a particular pressure level.