The word casual was first used in the 1370s, where it was spelled casuel. Two centuries later, it went through a brief phase when it was spelled casuall, and we've been writing it the modern way since the mid-seventeenth century. The adjective was taken from Middle French and traces to the Latin word causalis, meaning "by chance". This developed into the modern definition on a notion of something not happening with regularity, which later translated into informality. Causalis comes from the earlier Latin word casus, meaning "event", and that's from cadere, a verb for "fall" (at the time, things happening were likened to falling, similar to the word befall). Ultimately, cadere is reconstructed as coming from the Proto-Indo-European root kad, also "fall". Usage of the word casual has been pretty constant for about the last century.
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.