Abstract: This essay focuses on untranslatability to discuss the diachronic temporality of the history of concepts. Defining untranslatables as the paradoxical origin and product of translating, it explores their role in mediating the long-term history of concepts by disrupting the historical boundaries of a period and challenging the contexts through which past meaning is confined to the moyenne durée. Addressing first the critical appraisal of the history of ideas by Quentin Skinner and J. G. A. Pocock, it discusses their alternative suggestion of a history of discourses, rather than concepts or ideas, to move to Pocock’s formulation of the category of “diachronic translation” as a shift from the moyenne to the longue durée. It then turns to Begriffsgeschichte to explore the interrelation of untranslatables, Koselleck’s consideration of translation, and his theory of historical times. It suggests that Koselleck not only states that translation mediates the history of concepts, but also envisions a distinct temporality associated with the aporetic condition of translating what is untranslatable. The aporia of translations underlies both the historical depth of concepts as a conceptual reserve and an act of silencing past meaning. The ensuing conjunction of surplus and erasure qualifies Koselleck’s category of multiple times by designating the time of translation as “obscure time.” It is a time that displaces us from the apparent meaning of concepts in a certain period by receding toward the otherness of the past and suspending meaning that is already in the future. These two characteristics of obscure time, its receding and suspending nature, not only stand against the continuity of periodizing; they also make visible a politics of translation as an act of disruption of the present wherein the past becomes a reserve of meanings resisting appropriative interpretation.