An individual, organism, or new genetic character arising or resulting from mutation.
Slang. One that is suggestive of a genetic mutant, as in bizarre appearance or inaptitude.
The word 'mutant' is used in the text to describe someone with different religious beliefs, values, appearance and way of life then what the people of Waknuk believe 'god has chosen'. So say you get your hand amputated, you are now a mutant, while the real deffinition states you need to be born with that mutation. Another thing is the people of Waknuk believe if you're not the same as them, you are not normal. Which is not the case with real life because most people thrive on being unique and embrace difference.
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