From Longman Dictionary of Contemporary Englishpasserbypass‧er‧by /ˌpɑːsəˈbaɪ $ ˌpæsər-/ noun (plural passersby) [countable] TTRDsomeone who is walking past a place by chance They sell drinks to passersby.
Examples from the Corpus
passerby• The youngest died in the arms of a passerby, Boyle said.• A passerby said the noise was from a wedding celebration.• But the most haunting reminders of the massacre are the hope-filled faces of the young victims that stare out at every passerby.• But outside the battered congress building few passersby look twice at yet another standoff between demonstrators and riot police.• The robbery was witnessed by several passersby.• The cat jumped to the floor, darted through the legs of several passersby, and disappeared into the bar.• The attackers were apprehended by an unknown passerby, who chased the three youths and recovered the stolen bicycle. 