From the notion that if two sides of the same blade are sharp, it cuts both ways. The metaphor may have originated from the Arabic expression سَيْف ذُو حَدَيْن (sayf ḏū ḥadayn, “double-edged sword”). (In the Bible, the word of God is described as a being sharper than a double-edged sword.) The metaphor is first attested in English in the 15th century. It is not to be confused with a double-ended sword.
- (idiomatic) A benefit that is also a liability, or that carries some significant but non-obvious cost or risk.