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1870, from agnostic + -ism.
The agnostic does not simply say, "I do not know." He goes another step, and he says, with great emphasis, that you do not know. [Robert G. Ingersoll, "Reply to Dr. Lyman Abbott," 1890]
A denial of knowledge about whether there is or is not a God. An agnostic insists that it is impossible to prove that there is no God and impossible to prove that there is one. (Compare atheism.)