You counter the facetiousness in a way it stops spreading and possibly even spark a constructive discussion is how I understand it (ESL though). I certainly observed this phenomenon myself (although as the person being facetious, I often feel like "I was joking, I actually agree, that's indeed what I was actually implying, but good you made it clear and explicit I guess")

I guess you'd disarm the person being facecious rather than the facetiousness, like you'd disarm someone about to cast you a magic spell.

> I guess you'd disarm the person being facecious rather than the facetiousness

No, you disarm the facetiousness, the same way you'd disarm a trap. Disarming the person wouldn't make sense.

https://en.wiktionary.org/wiki/disarm

> 2. (transitive) To deprive of the means or the disposition to harm; to render harmless or innocuous.

> [quotations] to disarm a man's wrath

Interesting, in French, you can both disarm someone or a gun.

https://fr.m.wiktionary.org/wiki/d%C3%A9sarmer

You can disarm a person in English, but you can't disarm them of their mood.

In French neither, I took this as a figure of speech where facetiousness is likened to a weapon.