Words that are pluralized in the middle?
Why is the plural of "passerby" "passers-by" and not "passerbies"?
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I have no authorative source, but my common sense explanation is that the word is derived from the verb phrase
to pass by something. As the noun-constructing suffix got attached to the verb part
pass, making it
passer, it is logical to chain the plural suffix -s after the first suffix. The
by part is more of an additional particle for the word, I think it is even acceptable to write
passers-by with a dash.
A weak point in this explanation is that other nouns derived from compound verbs don't build a plural in the middle (compare
makeover). But those don't use a suffix to create the noun, so no noun-chaining can occur.