A plant (genus Rheum or similar) with large, thick leaves that resemble a dead or heavy tongue; a plant name from folk botany.
From 'dead' plus 'tongue,' combining the adjective with a body-part noun. Folk plant names often use body-part metaphors because their leaves or flowers resembled parts of the human body to herbalists.
Folk plant names are linguistic snapshots of how pre-scientific people understood nature—they named plants by what they looked like, so 'deadtongue' tells you that those thick, floppy leaves reminded someone of a lifeless tongue.
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