A person who passes by or walks past something, especially someone who just happens to be in a place without being part of what's happening there.
Compound word from 'passer' (one who passes, from Old French 'passer') and 'by,' literally meaning 'one who passes by.' It has been used this way since at least the 17th century.
The plural 'passersby' is tricky—many people say 'passerbys' but the original rule puts the 's' on the main noun 'passer,' showing how English plurals can surprise even native speakers!
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