Actively defecating or the act of elimination from the bowels; also used as a vulgar intensifier in speech.
From Old English 'scītan,' related to Proto-Germanic and Indo-European roots. The word has always been crude and direct, avoiding euphemism, which is why it remains marked as vulgar or informal in modern usage.
While it's considered crude today, 'shit' was once a straightforward, ordinary word in English with no particular shame attached—it only became taboo during the medieval period when polite society decided to hide bodily functions from speech.
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