As you've specified a weapon must be a thing, then trivially no, by definition.

But certainly information can be used for extortion, or to otherwise coerce people against their will, and maybe that's enough?

Are you sure that's the whole of the definition from the source you took it from? It looks suspiciously like a fragment of the definition on en.oxforddictionaries.com, omitting the part of the definition that is more general.

https://en.oxforddictionaries.com/definition/weapon

Yes. Consider a detailed blueprint for a physical weapon. Or information that make coordinated use of armed forces more efficient. Or blackmailing material.

I think information is likely more analogous to ammunition than to a weapon.

By itself, it is harmless, but when paired with an appropriate device, by a user with malicious intent, it can then be used to cause harm.

So psychological and material harm are irrelevant?