Originally, speakers used actual soap crates as stands for public speaking, hence the term 'soapbox.' Back in the day, anyone with an opinion and a soapbox could gather a crowd at the park.
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Literal Use
A 'soapbox' can be any temporary platform used for making an impromptu speech. He found an old crate and used it as a soapbox to address the passersby.
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Figuratively Speaking
Sometimes 'soapbox' is used figuratively to mean taking a stance on issues publicly. Whenever she gets on her soapbox, you know she's about to talk about environmental issues.