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The passerby did not stop to speak with him. She waved to the passerby.
The possessive form for the noun passerby is passerby's.
The possessive form of the singular noun passerby is passerby's.
example: That's a passerby's dog barking.
The plural form of "passerby" is "passersby."
I saw a passerby help an elderly person cross the street.