Languages exhibit striking semantic diversity, but different languages often share core metaphors. Conceptual Metaphor Theory (Lakoff & Johnson, 1980) claims that universal human experiences give rise to conceptual representations that are then expressed in language. But languages change slowly, making it difficult to observe implicit conceptualization affecting linguistic convention in real time. Here, we describe a shared conceptualization previously absent from speech that has now become conventionalized in linguistic metaphors. In two studies, we document how members of the US military talk about time using conventionalized lateral metaphors (e.g., ‘push the meeting right’ to mean ‘move the meeting later’). We show that military members, unlike civilians, consider such sentences to be acceptable—sometimes even more acceptable than more standard phrases. Moreover, military personnel seem unaware that these lateral metaphors are specific to their linguistic sub-community. Our findings suggest that implicit mental representations can become conventionalized metaphors in language.