Inframarginal externalities are externalities in which there is no benefit or loss to the marginal consumer. In other words, people neither gain nor lose anything at the margin, but benefits and costs do exist for those consumers within the given inframarginal range.
I think this is captured in the expressions “no skin off my nose” (you literally have no skin in the game!) or “I don’t have a dog in this fight”). More distantly (and I only repeat it because it’s such a good saying “not my monkey, not my circus” – a Polish expression meaning “not a problem I have any responsibility for”).