Upper Crust. Today used to mean upper class or wealthy individuals. History: When the pantler (the man in charge of cutting and serving the bread) was preparing the various breads for the lord's table, he would ensure that he served not only the tastiest of the breads, but also the cleanest undamaged part of the loaf: the upper Crust.
Curfew: Today a time limit to be inside. History: the name of a terracotta vessel that was placed over the embers at the end of the day to bank the coals. This made the fire easier to start in the morning. The name of this item was a couvre feu, or cover fire, which would be the last thing done before retiring for the night.
Red Letter Day: Today, a special day. History: In the illumination of Medieval calendars, reglgious feast days being highly important dates, were brought to the readers' attention the more readily by being picked out in red ink.
Rule of Thumb: Today, an accepted general average. History: It refers to the testing of the quality of milled flour by rubbing it between the thumb and forefinger.
Taking the gilt off the gingerbread: Today, removing the attraction, or illusion from something. History, this refers to the Medieval practice of covering gingerbread with a thin coating of edible gold leaf. Knock the gilt off, and you have plain brown cake.
A Hearty Trencherman: Someone with a large appetite. Historically, it took someone with a healthy appetite to eat not only the food served to him but also the bread trenchers on which it was served.