Medieval monks who had taken vows of silence developed a simple form of sign language to communicate (predating the development of modern sign language by centuries), and scholars know many of the signs they used.
Some examples of monastic sign language from the essay "Sign Language and Gestures in Medieval Europe: Monasteries, Courts of Justice, and Society," by August Nitschke:
I want to eat:
repeatedly move the first three fingers towards the mouth.
I want to drink:
place the tip of the thumb on the lips and tilt the fist like a bottle.
I am fasting:
press together the lips with thumb and forefinger
make a circle using both your thumbs and the fingers next to them.
place all the fingers of your right hand around the smallest finger of your left and stretch the latter, imitating someone who is milking.
Let your tongue protrude for a moment and move your fingers close as if you intended to lick them.
stretch our your hand, moving it as if turning the page of a book.
Make the sign for a book, and in addition make the sign of the cross.
make the sign for a book, and then make the sign of the cross on your forehead.
take the index finger, which has been placed below the eye, moving it downward twice.
raise one hand and move the slightly curved upper side of the fingers in a way that suggests a flying motion.
Blow on the tip of the forefinger, which is held pointing up.
Keeping the fingers together, move the right hand, keeping it straight, in front of the body in a zigzag maner like a swimming animal.
More info: wikipedia