The British poet and alchemist Thomas Norton used the word "attoms" in his 1477 poem, The Ordinal of Alchemy. Historian Howard Markel explains how Norton came to use the word, and points out earlier philosophers who raised the concept of indivisible units of matter.
http://www.npr.org/2010/11/19/131447080/science-diction-the-origin-of-the-word-atom?ft=1&f=1007