Defining "Atheism"

[Ryft]

Whenever someone responds ... with a "I don't know," it tells you nothing about their beliefs.

Precisely. It tells you about their knowledge; specifically, that on said issue their knowledge is either insufficient or non-existent ("I don't know"). What a person believes and what he knows are categorically different things. As I said, "I don't know" is an epistemic response, reflecting agnosticism (privation of gnosis, knowledge). It doesn't tell you whether they are an atheist or a theist ("it tells you nothing about their beliefs"), since both can answer "I don't know" to the question of God's existence; e.g., a fideist says, "I don't know that God exists but I believe he does."

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Technically speaking, I'm agnostic about an invisible, intangible leprechaun masturbating to an equally invisible, intangible issue of Playboy magazine inside my closet; but, I am still an amasturbatinginvisibleintangibleleprechauninmyclosetist, as I lack belief.

So in terms relevant to deity, "Technically speaking, I'm agnostic about God; but I am still an atheist, as I lack belief." That is, you employ the term agnostic in your atheism because each term addresses a different aspect—what you do or don't know on the one hand, and what you do or don't believe on the other.