I always thought this a funny name for a donut-shaped thing. Why is it called a washer? What gets washed???
Good question. I don’t have a definitive answer for you but the Online Etymology Dictionary says
[QUOTE=online etymology dictionary]
“flat ring for sealing joints or holding nuts,” 1346, generally considered an agent noun of wash (q.v.), but the sense connection is difficult, and the noun may derive instead from the ancestor of Fr. vis “screw, vise.”
[/quote]
I’m not quite sure what they mean by agent noun of wash.
In many states its a warsher anyhows…
I guess it’s all a wash! 
I’m not sure either, but if I had to guess an agent noun is a word that expresses someone who does something. A farmer works on a farm, a computer is something that computes, and so on. So a washer is something that washes in a strange sense of washes.
That’s actually a Staff Report by the lovely Una Persson. But thank you; it was very informative. I guess the origin of washer is lost to history.
According to the signature at the end, that’s a Staff Report by Una. So it’s “Mistress”.