http://www.straightdope.com/mailbag/mwasher.html
Why are washers called that? Using the Oxford English Dictionary (OED), my gal Fierra found that "washer," in the sense of "a perforated annular disc or flattened ring of metal, leather, or other material placed between two surfaces subject to rotative friction, to relieve friction and prevent lateral motion and unsteadiness," is "of doubtful origin." The earliest cite in the OED is from 1346 (spelled "whashers"), but the first quote that describes their use is from 1611:
. . . an iron hoope, amongst gunners called a washer, which serues to keepe the iron pin at the end of the axeltree from wearing the naue.