I am making this post because it seems there are still so many ignorant people in the world who enjoy trying to belittle and bully others. Maybe they genuinely don't know the racist origins for the reason redheads are often described as "Gingers" and so I will make its origin very clear here. If you choose to continue to use the word to describe people with red hair after knowing its true origin I can only come to the conclusion that you are racist against Celts.
Ginger is not describing a hair colour
Yes that is right, read it again and let that sink in. The spice ginger is a blonde colour, it is not red or anything like the colour of a redheads hair. If you were going to use a spice to describe the colour of a redhead's hair you would use cinnamon, paprika, turmeric or even chilli, but ginger! No, it doesn't make sense and there is a very good reason for that, it has absolutely nothing to do with hair colour.
The English looking down on Celts as sub-human
The English have a long history of being hateful and degrading towards those with Celtic origins, they described them as stupid drunks, brutes, insinuate they have low intelligence and make cartoon drawings of them depicted as apes, this was especially prevalent in the 1800s. It isn't so easily accepted or tolerated these days, but the hate crimes towards Celts still continue every time someone describes a redhead as a "ginger".
How did being Celtic become associated with the spice ginger?
In the early 1800s an English biscuit company who made ginger biscuits "ginger nuts" decided to depict a stereotypical looking Celt on the side of the tins. The Celt was wearing a green suit, had red hair and blue eyes, they named this character "John Ginger". The English then started to call Celts "ginger nuts" or "gingers" at the time it didn't matter what colour their hair was but later it became that because so many Celts do have red hair. It soon moved on to describe red-haired animals too, especially hens and cats. It was supposed to be derogatory and implying the person was a joke and not very intelligent.
This ingrained a deep hatred towards those with the most Celtic looks, and the most Celtic look of all is red hair. England to this day is still awash with hatred and bullying towards those with red hair. Constantly called a ginger and taunted endlessly teenagers have committed suicide over it, people have been beaten up, purely because of the colour of their hair and families forced out their homes because the locals were throwing bricks at their homes and constantly abusing the entire family because they have red hair.
If this isn't a hate crime, what is?
Because it is so prevalent in England it is now treated as a hate crime, however, this doesn't stop ignorant people going about throwing the word at people with red hair and thinking it is ever so funny. If you do that you are nothing but a racist bigot.
Some examples:
http://news.bbc.co.uk/2/hi/uk_news/england/tyne/6714735.stm
https://www.dailymail.co.uk/news/article-2514859/Helena-Farrell-15-killed-bullied-ginger.html
https://www.dailyecho.co.uk/news/9507952.beaten-up-for-having-ginger-hair/
https://www.telegraph.co.uk/news/celebritynews/9788039/Bullied-and-worse-for-being-ginger.html
This is just a small sample, you will find hundreds more if you look.