It's not common, or even used at all as a slur nowadays. People assume it's a racial slur against Asians, sense in WWII, the US often depicted the Japanese as animals in their propaganda. Rats seemingly became common because of the tagline "Trap the Jap", and because Asian caricatures are often depicted with large teeth.Rat is a racial slur against Asians? Is this common? I don't think I have ever heard of it. Some regional thing this snowflake thinks everyone knows about?