Pneumonoultramicroscopicsilicovolcanoconiosis

Pneumono­ultra­micro­scopic­silico­volcano­coniosis ( NEW-mə-noh-ultrə-my-krə-SKOP-ik-SIL-ih-koh-vol-KAY-noh-koh-nee-OH-sis) is a 45-letter word coined in 1935 by Everett M. Smith, the then-president of the National Puzzlers' League. It has sometimes been used as a synonym for the occupational disease known as silicosis; however, this term is incorrect, as most silicosis is not related to inhalation of volcanic dusts.

Source: Wikipedia — Pneumonoultramicroscopicsilicovolcanoconiosis (CC BY-SA 4.0)