"fender(n)
late 13c., shortening of defender. Used of attachments to boats at first, of fireplaces since 1680s; application to automobiles is 1919."

-http://www.etymonline.com

I also call them "mudguards" since a Welsh associate explained that "mud" is a British euphemism from the days when horse drawn vehicles were common on roadways. I live in a fairly rural area with working ranches & farms & "low water crossings" to cycle though after the rains. Planet Bike Mudguards for me.

-Bandera