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Arma virumque cano, Troiae qui primus...

vis, vis is an irregular "very i-stem" 3rd-declension noun. In the singular, it is declined vis, vis, vī, vim, vī, and in the singular it means force, like the English word vim (with vim and vigor"). In the plural, it has a different base: virēs, virium (still i-stem), vitibus, virēs, viribus, and in the plural it means strength, as in the English expression, "with all my strength."