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.”
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 logically used. But it fails, for it is illogical to claim you are one thing when you are really another. For example, what do you call at person who holds they know there is no God? According to the ‘lack of belief’ folks, they are atheist. But if a theist uses this same definition of atheism, they are wrong. Words to not change meaning based on who is defining the term.