(adj.) bland and clichéd; unexceptional
The reason we call things banal is because in medieval France, people paid fees known as bans for the use of communal equipment like millstones and ovens.
Can’t see a connection there? Well, these communal items were widely available to everyone (or at least, everyone who could afford to pay the local mill-owner or landowner the ban required). As a result, a related adjective banel emerged in Old French that described anything that was similarly commonplace or widespread in its use or accessibility. From there the sense of banel gradually altered to come to describe anything ordinary, unexceptional, and ultimately trite, tired, and frequently encountered—and from there, we earned the word banal.
This etymology sparked a couple of questions on Twitter, wondering whether these bans were the same as matrimonial banns. A nice idea—and correct too! Despite that curious double-N spelling, marriage banns derive from the French word ban too: as well as referring to this payment, ban also meant ‘proclamation’, ‘legal prerogative’, ‘summons’, or ‘decree’, and because a legal edict like this was required to seal a marriage in the eyes of the law, the word simply transferred across.
There is a question mark hanging over whether these banns came directly from France into English, or have even earlier Frankish roots (which would likely place their origin in our language with the Anglo-Saxons before they even arrived in what is now England). But this is a difficult puzzle to solve as ban, or some word like it, is common to a host of continental languages in Europe and it’s all but impossible to single out precisely which one we took it from. Nevertheless, all these bans, banns and banals are etymologically the same.