In rhetorical and rationalist circles, a “Shill Gambit” is a logically fallacious attack through which the attacker attempts to dismiss everything their target is arguing by implying (or outright stating) that they are being paid by someone (or some entity) that would benefit from the point they’re making.
The aim is to make the other person’s argument seem less convincing or completely moot by suggesting they are only saying what they’re saying because they’re being paid to do so.
This is a version of what’s called an ad hominem attack, which means attacking the person making the argument, rather than the argument itself.
And often this sort of attack is leveraged when the attacker doesn’t have a good counter to a point that’s been made, or when they want to poison the well of the conversation, making it seem somehow unworthy of debate because those on the other side are doing something wrong or latently dismissible.
There are times when a conflict of interest is a legitimate piece of data to introduce into a conversation or debate, of course.
When someone is actually being paid by an oil company to question the global scientific consensus of climate change, for instance, we might rightly question the validity of their arguments based on the conflicted interests of their source (which in this case would be the folks paying them).
This sort of claim becomes fallacious, however, when such accusations are unbacked or irrelevant.
Claiming that someone supports vaccinations because they’re a medical professional, for instance, might superficially seem like the same thing (they work in a profession that benefits when medical services are rendered, after all!), but unless the person engaging in said support works directly for a vaccine-maker, this “guilt by association” accusation falls flat—the incentives are far different, and the preponderance of external evidence isn’t slanted against them from the get-go.
While such well-poisoning tactics might serve (at times quite effectively) to demonize people or groups within a specific sub-culture, these accusations seldom carry much weight beyond the confines of shared, sub-cultural alternative realities because they fail to address the arguments themselves: folks who don’t belong to the same multiverse of assumptions (and ideological partisanship) will tend to see these claims as flat and baseless.