Websters: something (as a club, knife, or gun) used to injure, defeat, or destroy.
I'll add that this is generally against a person or persons. So my gun sitting on the table needing cleaned after a plinkin session is not a weapon, nor is the pen on my desk or my daughters softball bat on the porch. Only when I train, use or carry for SD a gun for the above definition does it become a weapon. I call a hammer a hammer, a bat a bat, a knife a knife, a bow a bow, a gun a gun.
Rule #2: Never let the muzzle cover anything you are not willing to destroy. It seems that if you follow the 4 rules a gun is always a weapon by the definition you've provided.
While I understand the argument people are trying to make about only calling it a weapon when it's being used as such, this does not get around the fact that guns are inherently weapons, unlike a piece of cake or a pie.
This doesn't mean that you have to go around calling firearms weapons all the time, and in all circumstances. However, this is not the idea that the OP was contending.
We need to control the language to put out a better public image.
What should we always call firearms? Call them just that "firearms", guns? Call it what it is, rifle/pistol/shotgun?
I like "defensive firearm". Think of it as the opposite of "assault rifle". The media pounded on the term assault rifle and altered the meaning and perception. They controlled the language.
Imagine if we always described a weapon...er...a firearm...in a language "positive" way. If we tie the word defense to the word gun or firearm, that would be golden.
The OP is suggesting we as firearms/weapons/<insert subtype here> owners play the same game as the other side and add spin to the words we use. Trying to use smoke and mirrors does us no good for various reasons. In part , if we were to make a concerted effort as a community to disavow the idea that firearms are weapons by refusing to use that word to describe them, it hinders the idea of many 2A arguments that firearms are for self-defense and not only "sporting purposes".
It's one thing what you call them in everyday life, but playing political spin games and denying a core aspect of what firearms are is another.