knee-plate

from The Century Dictionary.

• noun In ship-building, a small triangular plate fitted at the point of connection of the beam and frame to strengthen it.
• noun A defensive appliance for the tilt used in the fifteenth and sixteenth centuries, consisting of a broad steel plate shaped to cover the thigh and to project on each side. Its chief object was to protect the left leg from friction against the barrier.
• noun A similar defense shown in pictures of the sixteenth century as worn over the right leg.