Everett Smith

Everett Smith is a puzzler who, in 1935, may have coined the word pneumonoultramicroscopicsilicovolcanoconiosis (also spelled pneumonoultramicroscopicsilicovolcanokoniosis) as a hypothetical long word that could result from the protraction of medical terms. It is defined as "a lung disease caused by the inhalation of very fine silica or quartz dust." The actual name of the disease is silicosis.

At 45 letters, it is certainly the longest word ever to appear in a non-technical dictionary of English (Source: Oxford English Dictionary).