Comparative form of dirty: more covered in dirt, mud, or grime, or more morally wrong and unfair.
From Old English 'drit' (excrement), evolved to mean any unwanted substance; 'dirtier' is the comparative form showing greater degree.
The phrase 'playing dirty' in sports or politics has deep roots in 19th-century Victorian morality where 'dirty' was synonymous with 'unchaste'—showing how a single word carried both physical and moral weight.
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