I think that is actually "Can't win an argument, cast an insult." Racist is seen as an insult sometimes. But, you could use idiot, fool, moron, dumbass etc., and still show that you have no retort... just anger and insult. Idiot, moron, fool are all indistinct (although there may be an official hierarchy somewhere for them). But, "Racist" is distinct. A racist is a specific thing. I think everyone is racist and tribal to some extent. We're all taught that from birth, on some level. Some of us are more racist and tribal than others. Some of us deny it, some of us embrace it, some of us work to erase it from ourselves. The ones that embrace it and are proud of it tend to be seen as really racist by everyone else.