Agnosticism comes in a weaker and a stronger version, and they are not small variations on each other.
The weak or temporary form holds that whether any god exists is not currently known. It leaves open that evidence or argument might one day settle it, and it is compatible with active inquiry. The strong or permanent form holds that the question cannot be settled by any human being at all, because its object lies outside the reach of experience and no possible observation would bear on it. On the strong view the shortage is not of evidence but of any procedure that could produce evidence. Thomas Henry Huxley's own statements sit nearer the strong end for a particular class of questions, though he framed his position as a method rather than a doctrine and resisted turning it into a creed.
The strong form has philosophical ancestry. Kant argued that reason cannot reach past possible experience to prove or disprove the existence of God, and that the traditional proofs fail for that structural reason rather than through any fixable defect. Herbert Spencer, writing in the decade in which the word was coined, built a system around the Unknowable, an ultimate reality of which nothing determinate can be said.
There is a further step that agnosticism itself does not take. Theological noncognitivism, sometimes called ignosticism and usually credited to the rabbi Sherwin Wine, holds that the question is not merely unanswerable but defective: that the term God, as ordinarily offered, is too under-specified for the sentence God exists to carry a truth value at all. On that view the agnostic has already conceded too much by treating the proposition as a genuine one awaiting evidence, and the right response is to ask for a definition before anything else. Whether this is a deeper form of agnosticism or a rejection of it is itself argued about.