On Linus Torvalds, technical and corporate communications
ds9a.nl
ds9a.nl
The only thing you don't need to do is swear and the only thing you need to really add is a better solution:
"That's a terrible terrible idea! It's bad design because of x, y and z. Instead, why don't we do a? We can write tests for it and we'll need to refactor the module, but it's a clean solution. We could also do b as a quick fix. I mean if we have to, we can do c, but I think we should try to do a."
You should not be afraid to say ideas are terrible. On a corporate team, just don't swear (in slack, you can in person. It's about professionalism) but if you say something is terrible, say why, defend your argument and be prepared to offer a better solution.
I see a couple of issues with this.
First, the initial sentence doesn't do any useful work. You're already explaining the problems with the idea. That's all you need to do.
Second, you're writing from the assumption that you're definitely right. It might turn out that this actually isn't such a terrible idea for reasons that you haven't thought of. But now that you've said, in such definitive terms, that it's a terrible idea, it will be difficult for you to backtrack if you turn out to be mistaken.