pneumonoultramicroscopicsilicovolcanoconiosis (n.) Look up pneumonoultramicroscopicsilicovolcanoconiosis at Dictionary.com
1962, "A facetious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust' but occurring chiefly as an instance of a very long word" [OED]. Said in an early reference to have been invented by seventh grade students in Norfolk, Virginia.