expression, godan verb (-su)
hone a blade; sharpen a sword
Literal meaning: to sharpen a sword or blade, especially in a traditional context.
刀鍛冶が寝刃を合わすのに何時間もかけた。
The swordsmith spent hours honing the blade.
Figurative use: to secretly plan something, often with a negative connotation.
彼らは裏で寝刃を合わしているらしい。
They seem to be scheming behind the scenes.