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]
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.
1610s, "godless, impious," from Latin atheus, from Greek atheos, from a- "not, without" (see a- (3)) + theos "god" (from PIE root *dhes-, forming words for religious concepts). From 1880 as "not considering the existence of God, having no reference to God, irrespective of divine existence or power."
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Definitions of atheistic from WordNet
rejecting any belief in gods;
Synonyms: atheistical / unbelieving
related to or characterized by or given to atheism;