Why is the term "double-edged sword" used for something that can be favorable and unfavorable?

Solution 1:

Some people believe that a two-edged sword is more dangerous to its user than a single-edged one, but my experience (in martial arts) does not concur. It's not likely that a skilled swordsman is going to hurt himself with the reverse edge.

A two-edged sword is designed to be more dangerous to the target, not the wielder, by cutting on both the forward stroke and the back stroke. This idea is consistent with some of the earlier uses of the phrase:

The burden of taxes, like a two-edged sword, reduced men to poverty, and exposed them to be seduced by bribery. (1809)

In this sense, it is likened to the phrase: "cuts both ways" - referring again to the two sides of the sword stroke.

I don't know at what point "cuts both ways" and "two-edged sword" came to have the current meaning of good and bad, instead of just bad and worse, but I expect the two phrases evolved together.

Solution 2:

Double-edged sword is somewhat of an imperfect metaphor, used with decidedly more of a semantic emphasis on double-edged than on sword. In other words, the poetic implication of cutting both ways supersedes the historical reality of the actual weapon.

Solution 3:

"Double-edged sword", as a metaphor, has always been linked with "cuts both ways", meaning it can (figuratively) hurt both the person attacked and the attacker.

From 1793:

Mr. Burr (as was supposed) was too sore to be unbiassed ; he has, therefore, delivered an opinion which, like a two-edged sword, cuts both ways, for he declares that there was no sheriff, which, if admitted, destroys the legality of the votes and casts an odium on the Governor for suffering so important an office to be vacant.

From 1713:

This sort of argument is very unfair. It is also dangerous to to the cause of those who introduce it. It is a sword which cuts both ways. For we find the House of Commons has been more guilty than the Lords in this respect.

The metaphor has never, for at least the past 300 years, been used in reference to a "real" sword fight, but always rather invoking the image of a blade that can do damage to the person wielding it, in addition to injuring the attacked party.

And it's interesting to note that, in the past 100 years, "cuts both ways" has attained a life of its own, becoming much more popular than "double-edged sword" and its kin.

Also note, however, that "two-edged sword" (which has always, up until 20 years ago, been more popular than "double-edged sword") achieved a high degree of popularity in the early 1800s, in a religious context that is apparently unrelated to the metaphor.

Solution 4:

A double edged sword hurts the user and the victim, even though more realistically it should be a double-ended sword.

"Everything is a double-edged sword. Nothing isn’t a double edged sword. Even a single-edged sword is a double-edged sword. Because you can cut something with it, but the other end is kind of flat and doesn’t really cut well. It’s kind of a double-edged sword." - Louis CK

Or a single-edged sword. Kill on one edge, fail on the other (which, ifsword fighting someone, will kill you)