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Now, he said “Sick ’em boy”, but what I heard was, “Chopper, sick balls.”
— Narrator • Stand By Me

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The narrator distinguishes the command actually spoken from the much more alarming command they heard. A general instruction to attack becomes a specifically bodily threat in the retelling. That difference gives the line its humour, turning a small verbal change into a sharp escalation of danger.
Context reviewed