I read Post Office by Charles Bukovski and I came across this sentence:
The soup was a bullneck named Jonstone.
Who could help me to understand this soup meaning
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This is the third time "soup" has been used in this context in this novel. The narrator is a temporary Christmas postman and he says:
They only gave you a block or two and if you managed to finish ... the soup would give you another.
... I was—a substitute mail carrier... the soup was easy and I strolled around doing a block or two...
and finally, your example "the soup was a bullneck named Johnson".
So a "soup" is a person, this person can be tough (a bullneck) or "easy". And if you finish your work early, this person can give you extra work.
So "soup" must mean "superintendent" or "supervisor", I.e the person in charge of the mail carriers. It is an abbreviation, but not one that I've seen before. However, the meaning is clear enough in context.