1570s, "godless person, one who denies the existence of a supreme, intelligent being to whom moral obligation is due," from French athéiste (16c.), from Greek atheos "without god, denying the gods; abandoned of the gods; godless, ungodly," from a- "without" (see a- (3)) + theos "a god" (from PIE root *dhes-, forming words for religious concepts).
The existence of a world without God seems to me less absurd than the presence of a God, existing in all his perfection, creating an imperfect man in order to make him run the risk of Hell. [Armand Salacrou, "Certitudes et incertitudes," 1943]
1540s, "daring, defiance, boasting," from French braverie, from braver "to brave" (see brave (adj.)) or else from cognate Italian braveria, from bravare.
No Man is an Atheist, however he pretend it and serve the Company with his Braveries. [Donne, 1631]
The original deprecatory sense is obsolete; as a good quality attested perhaps from 1580s, but it is not always possible to distinguish the senses. Meaning "fine clothes, showiness" is from 1560s and holds the older notion of ostentatious pretense.
"the doctrine that there is no God;" "disbelief in any regularity in the universe to which man must conform himself under penalties" [J.R. Seeley, "Natural Religion," 1882], 1580s, from French athéisme (16c.), with -ism + Greek atheos "without a god, denying the gods," from a- "without" (see a- (3)) + theos "a god" (from PIE root *dhes-, forming words for religious concepts). A slightly earlier form is represented by atheonism (1530s) which is perhaps from Italian atheo "atheist." The ancient Greek noun was atheotes "ungodliness."
In late 19c. sometimes further distinguished into secondary senses "The denial of theism, that is, of the doctrine that the great first cause is a supreme, intelligent, righteous person" [Century Dictionary, 1897] and "practical indifference to and disregard of God, godlessness."
In the first sense above given, atheism is to be discriminated from pantheism, which denies the personality of God, and from agnosticism, which denies the possibility of positive knowledge concerning him. In the second sense, atheism includes both pantheism and agnosticism. [Century Dictionary]