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.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.