From Online Etymology Dictionary:
Atheist, 1570s, from Fr. athéiste (16c.), from Gk. atheos “to deny the gods, godless,” from a- “without” + theos “a god.”
Agnostic, 1870, from Gk. agnostos “unknown, unknowable,” from a- “not” + gnostos “(to be) known.”
Many atheists pride themselves that their views are based on knowledge and not beliefs. While they claim there is no good evidence for God’s existence, neither can they say they know God does not exist. To know God does not exist would require all knowledge of existence, which no person has. To accept the badge of agnostic is in conflict with the atheist’s claim that they only hold views based on knowing something. So to avoid calling themselves agnostic, the definition ‘lack of belief in God’ has come about so the term atheist can still be without acknowledging it is a belief.