You reach out for the thing and your hand closes on nothing. There is no matter there: nothing to hold, nothing to weigh. Put that hand on a set of scales instead and the needle does not move — and that is where the second use begins, with something that adds no weight to the question being decided.
From the word’s history
From Latin im- (not) plus materialis, from materia (matter, material), through Medieval Latin immaterialis and Middle French immateriel. The literal reading, not made of matter, is the older one; the source marks the not-important uses as figurative.
拆開來是 im-（不）加上 material（物質的）。material 來自拉丁文 māteria（物質、材料），再經 immāteriālis 和中古法文 immateriel 進入英文。字面的「沒有物質」是比較早的一層；我們手上的資料把「不重要」那幾個意思標成 figurative（比喻用法），所以兩者的先後關係是有出處的，不是我們自己推的。
Wiktionary entry for immaterial (kaikki extract supplied in this packet): from Medieval Latin immaterialis, from Latin im- + materialis, from materia; the unimportance senses are tagged figurative.
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