h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I suppose it's a kind of "Ad Hominem".

The great writer and essayist C.S.Lewis wrote about a kind of logical fallacy which he nicknamed "Bulverism" (from its notional discoverer Ezekiel Bulver in which he heard his mother exclaim:

"You say that becuase you're a man!"

In that moment, the young man realised that "Contradiction is no necessary part of argument" and that it in such cases you need only show that your opponent has his own self-interest at heart, or some similar reason for upholding their side of the argument.

Eg: "The right honourable Mr Smith MP is only proposing this law becuase he has shares in Such-and-such Corp Ltd."
(In fact this is entirely orthongonal to the argument, he may stand to gain from the law being passed, and he may not; but that doesn't make it a good or bad law.)