dull sword; blunt sword
Literally a sword that is not sharp. Can also be used figuratively for a slow-witted person or ineffective method, though that usage is less common.
鈍刀では硬いものを切るのは難しい。
It's hard to cut hard things with a dull sword.
彼の議論は鈍刀の
His argument lacks sharpness, like a blunt sword.
Compound of 鈍 (don, 'dull') and 刀 (tō, 'sword'). The reading is on'yomi for both characters.