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In page Siberia:

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Some sources say that "Siberia" originates from the Siberian Tatar word for 'sleeping land' (Sib-ir), but this discourse does not correspond to the actual Siberian Tatar language.[4] Mongolist György Kara posits that the toponym Siberia is derived from a Mongolic word sibir, cognate with modern Buryat sheber 'dense forest'.[5] A different hypothesis claims that the region was named after the Sibe people.[6] Another account sees the name as the ancient tribal ethnonym of the Sihirtia or Sirtya (also Sypyr [sʲɵpᵻr])), a hypothetical Paleo-Asiatic ethnic group assimilated by the Nenets.[citation needed]