washer "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." http://www.etymonline.com/index.php?search=washer&searchmode=none This a great site for word origins.

1) "I think an equally valid question is why a disk with a hole in it is called a washer. What exactly does it wash?" "I agree completely on the word washer... To get off topic a bit, just for kicks, I looked up the etymology... Looks like no one else knows why it's called a washer either, though several sources I checked did agree that it likely originated with "someone or thing that washes". None knew, however, the logic that takes "washing" to disk with a hole in it." Source and further information: http://www.home-barista.com/forums/versalab-m3-grinder-t408-20.html 2) Websters Dictionary indicates the same etymology for this mechanical washer and for instance, the dish washer: http://cancerweb.ncl.ac.uk/cgi-bin/omd?query=washer&action=Search+OMD This does not explain either why the device became that name. 3) Here a list with diverse definitions. Interessant are the translations in various languages. The washer that washes is usually called with some word built on the word for washing in most languages. The round flat disk uses such a name only in English. "ˈwasher2 noun a flat ring of rubber, metal etc to keep nuts or joints tight Example: Our tap needs a new washer. Arabic: Ø­ÙŽÙ„ÙŽÙ‚ÙŽØ©ÙŒ جِلْدِيَّه أو مَعْدَنِيَّه Chinese (Simplified): 垫圈 Chinese (Traditional): 墊圈 Czech: tÄ›snÄ›ní Danish: pakning Dutch: leertje Estonian: tihend, seib Finnish: tiivistysrengas French: rondelle German: der Dichtungsring Greek: ροδέλα Hungarian: alátét Icelandic: skinna, skífa Indonesian: ring Italian: rondella Japanese: 座金 Korean: (너트 등의) 와셔, 고리쇠 Latvian: blÄ«ve Lithuanian: tarpiklis Norwegian: pakning Polish: podkÅ‚adka, uszczelka Portuguese (Brazil): arruela Portuguese (Portugal): anilha Romanian: ÅŸaibă Russian: прокладка Slovak: tesnenie Slovenian: tesnilo Spanish: arandela, junta Swedish: packning Turkish: pul, rondelâ" http://dictionary.reference.com/browse/washer