Remember when Mistress Quickly (disguised as the "Fairy Queen") sings a song designed to scare the you-know-what out of Falstaff in Act 5, scene 5? Well, during that freaky song she orders her little "fairies" to go over to Windsor Castle and get it ready for the Order of the Garter (5.5.70). She tells them to spruce up the joint and even asks them to spell out the order's motto with a bunch of flower petals (5.5).
The motto? "Honi soit qui mal y pense" or, "shame to him who thinks evil of it." Modern translation: "shame on anyone with a dirty mind." (More on this in a minute, kids.)
Okay. What is this Order of the Garter and, more importantly, why do we even care?
Basically, the Knights of the Garter is the oldest order of knighthood in England and it's reserved for the cream of the crop. You can only become a member if the monarch invites you to join.
In Shakespeare's day, that was Queen Elizabeth I, whose nickname, we'd like to point out, was the "Fairy Queen." Every year there was a big ceremony called the Garter Feast, where new members were inducted. Of course, this all went down in a chapel inside, you guessed it, Windsor Castle. (You know, that castle that's sort of looming in the background during the entire play?)
Obviously, a disgraceful knight like Falstaff would never, ever be allowed to join this club. That's why the shout-out to the exclusive order is so fitting, right? Also, the order's motto seems like a pretty great way to summarize the moral of this play: shame on Falstaff for having such a dirty mind. What's more, all of this has led some scholars think that Shakespeare wrote Merry Wives as an entertainment piece for the Garter Feast of 1597 (source).