infragilis

in-frăgĭlis, e, adj., that cannot be broken to pieces.

I Lit.: adamas, Plin. H. N. 20 prooem. § 2.—

II Trop., strong , that cannot be weakened : vox, Ov. Tr. 1, 5, 53: animi, Sen. Vit. B. 9.