Extremely inexpensive; available at a very low price.
This phrase emerged in early 19th-century America, playing on the idea that dirt or soil is so common and abundant that it has virtually no monetary value. The expression reflects agricultural societies where land and soil were fundamental but often taken for granted as essentially free resources.
The phrase reveals cultural attitudes about value and abundance - dirt is literally everywhere and costs nothing to obtain, making it the perfect metaphor for something worthless or nearly free. Interestingly, in modern times, quality soil can actually be quite expensive, making this phrase somewhat ironic in agricultural contexts.
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