fender

Fender is simply fend plus the agent suffix -er, which marks someone or something that does the action of the verb. That suffix has a long pedigree, descending through Old English -ere and West Germanic -ārī from Latin -ārius and ultimately Proto-Indo-European *-yósder.

The word stayed close to its roots: fend meant to ward off or defend, and adding -er simply named the thing that does that warding — whether a metal guard on a car or a bumper on a boat.

Because -er here is the same suffix seen in countless English agent nouns (baker, teacher, and so on), fender belongs to that huge family of 'doer' words built the same way.