"Atheist1570s, from Fr. athéiste (16c.), from Gk. atheos "to deny the gods, godless," from a- "without" + theos "a god". A slightly earlier form is represented by atheonism (1530s) which is perhaps from It. atheo "atheist."'
It appears that this definition doesn't tell the whole story. I won't pretend to fully understand the long and confusing history of the term atheist. However, here is something interesting from Wikipedia,
"The term atheist (from Fr. athée), in the sense of "one who denies or disbelieves the existence of God", predates atheism in English, being first attested in about 1571. Atheist as a label of practical godlessness was used at least as early as 1577."
It would appear that the 1571 definition is the original definition in English and it is remarkably similar to how most atheist describe themselves today. It's interesting that the word began to have a more negative connotation in 1577. I expect it was probably theists who began to gradually alter the impression of the word to further their agenda and criticize those who weren't in their club.