Well, it is Valentine's Day tomorrow. Mr. Marimo just looked out of his little glass house and realized this because he saw a few passerbys (or is it passersby?!?) carrying their valentines to the mailbox to mail.
Postponing deciding what he might do to celebrate Valentine's, Mr. Marimo decided to first look up this peculiar word. It turns out that the correct usage is passersby, but more and more people are using the term "passerbys" and so the word might eventually be an accepted form.
What is up with this strange word? Let's say you have a person who wants to pass by you. So, that person would be called a passby-er. However, when this word was originating in 1560 to 70 (according to dictionary.com*), the people of that time decided it was better to postpose the particle. In other words, they placed the "by" at the end of the word and so we now have passerby as the word we use today. Its plural form applies an "s" to the noun "passer" not to the particle "by".
* passerby. Dictionary.com. Dictionary.com Unabridged. Random House, Inc. http://www.dictionary.com/browse/passerby (accessed: February 13, 2018).