Agnostic was coined, by Huxley's own account, at the Metaphysical Society, the extraordinary London dinner club founded in 1869 where bishops, Catholics, positivists and scientists read papers at each other across the century's widest gulfs. Finding every other member equipped with an ism while he alone had, as he put it, no label to hang on his back, he invented one, suggestively antithetic to the gnostic of church history who professed to know so much about the very things of which he was ignorant. The word named a method before it named a mood: Huxley's agnosticism was the principle that it is wrong to say one knows the truth of a proposition unless one can produce evidence that logically justifies the certainty, applied without exception to theology, and equally, he insisted, to dogmatic atheism, which claims a knowledge of its own. He spent years defending and refining the term in essays against critics who called it atheism in a frock coat, and its success exceeded any polemic: within a generation it named the respectable middle station of Victorian doubt, and it still does the same work.
The setting mattered. The Metaphysical Society's procedure, courteous single combat over stated propositions, was Huxley's natural element, and the coinage is best read as a rule of that game raised into a philosophy of assent: state what you can prove, confess what you cannot, and treat the confession as honesty rather than defeat.