Recently, I've been catching weekday matinees at a nearby cineplex (rewatching! Hail Caesar! four times!) One unfortunate side effect of this indulgence is being forced to endure the same trailers over and over. In one of them, a ghastly summer comedy, a person with red hair is disparagingly referred to as a "ginger." Switch the first 'g' with the 'n' and you'll get the full import of the way the word was used in context.
I had first become aware of this peculiar prejudice many years ago. The otherwise intelligent and enlightened individual I was living with at the time was trying to explain why she thought The Pickwick Papers was funny. In one of the episodes, a character cast a slur on a red-haired person. I thought that Dickens was using the incident to show the ignorance and meanness of a social stratum. My companion found it to be hilarious and she judged my sense of humor to be somewhat retarded. To denigrate someone for their hair color was cruel and dumb then, I am even more disturbed by its usage now.
BTW, Hail Caesar! did feature a red-haired person. But not as a slur: