Dirty Chai extends Chai's assertion library to offer more expressive and readable syntax, particularly for negative assertions. Rather than expect(foo).to.not.equal(bar), you can write expect(foo).to.not.be.equal.to(bar). It enhances readability by allowing chained assertions with synonyms and implicit subjects, making tests easier to understand and maintain. It focuses on delivering more fluent and natural-sounding test assertions in JavaScript.