To defecate or to dirty, primarily used in Scottish dialect.
From Scottish Lowland English, possibly imitative or related to sounds associated with the described action. Regional dialects often preserve crude or scatological terms that disappeared from standard English.
Scottish dialect preserved this word while standard English sanitized its vocabulary—dialects are language's attic, preserving words and pronunciations that reveal how people really spoke centuries ago, before 'polite society' edited them out.
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