Definition of "unbendable"

No one would have originated the idea of piecing together several bits of hard unbendable material, and giving them elasticity by means of sinews or hide, unless they had previously been acquainted with the use of the plain bow.

1890, Journal of the Royal Anthropological Institute of Great Britain, volume 19

It takes expert, trained hands to handle a fracture; a board or thick, unbendable material under or next to the fractured part should be applied as a splint before the part is moved.

1960, Florence Dakin, Ella M. Thompson, Simplified Nursing: The Essentials of Practical Nursing

Or at least some hard, unbendable material. Steel? Or something even more sturdy? Some new high-tech material that was indestructible? Matt wasn't sure.

2003, Beverly Barton, The Princess's Bodyguard