To roll around in mud or water, or metaphorically, to spend a lot of time absorbed in a negative emotion like sadness or self-pity.
From Old English 'wealwian,' meaning 'to roll,' related to German 'wälzen.' The physical meaning of rolling in mud came first, with the emotional meaning of 'indulging in despair' emerging later as a metaphorical extension.
Pigs and elephants literally need to wallow in mud for survival—it regulates their body temperature and protects their skin from the sun and insects—so the word originally had nothing to do with emotion or weakness. But as humans became more sophisticated, we started using animal behavior as insults, so 'wallowing' in sadness became a moral criticism, turning a basic survival need into a character flaw when applied to humans.
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