dull blade; blunt sword
Refers to a blade that has lost its sharpness, often used in contexts of swords or knives. Somewhat literary or traditional in feel.
A dull blade cannot cut the enemy.
This sword is blunt, so it must be sharpened.
Compound of 寝 (ne, 'sleep') and 刃 (ha, 'blade'), with rendaku giving ば. Literally 'sleeping blade', suggesting a blade that has been idle and lost its edge. The exact historical derivation is uncertain, but the imagery is conventional.