The first attestations of the word platitude as meaning "an unoriginal remark" come from the early 1800s, but for a century prior to that the term existed as a more general adjective describing "the quality of being dull". In the original French, the word meant "flatness" because flat things were considered dull. The root in platitude is Old French plat, meaning "flat"; -itude is from Latin tudo, which signified a state or condition (and is from Proto-Indo-European tus). You may recognize plat from words such as plateau and plate. It is reconstructed as deriving from another Proto-Indo-European root which also sounded like plat and meant "flat". Usage of the word platitude over time peaked in the 1920s and has been decreasing since, as competitor cliche surpassed it in utilization.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.