It appears that in particular men were less prudish in appearing in it during the middle ages and there are numerous images of farmworkers tilling fields wearing little more than their shirt and braies. Images of people swimming in the 15th century depicted short braies tied at the waist. There is a least one image of men's underwear that takes the form of a bikini bottom tied on one side with cords.
The chemise was probably considered underwear for the woman, and it was considered improper for her to appear in only a chemise. It appears that in most times and places women did not wear any additional underwear under their chemise, probably the origin of jokes about who wears the pants in the family. Corsets (generally worn over a chemise) may also be considered undergarments, as they are not expected to be seen by the public.