When the weather is so hot that it enrages you. Especially prevalent in the UK where air conditioning units are mythical creatures that you only ever read about in books.
Examples of heat rage:
"I want to work out, but it's so damn hot and stuffy in my room that I don't feel like it. F*ck this weather!"
"Walking up this hill is making me sweat like crazy. I hate summer!"
by ThohtSlayer9000 July 21, 2021
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