"Humor" has been defined as the sudden occurrence of the unexpected. A funny (humorous) plural, it follows by logologic, is a plural that ends with an unexpected letter. Since most plurals in English are formed by adding an S to the singular, plurals that end with other letters are funny (strange).
Word Ways: Vol. 25
, Article 10.
Available at: http://digitalcommons.butler.edu/wordways/vol25/iss1/10