In medieval times, was it considered rude to put your elbows on the table?
In lower society, table manners practically did not exist in Medieval times and it was common for people to eat with their hands. Even the use of a fork was not common until the late Medieval period and then even only in the higher echelons of society.
The origin of placing elbows on the table seems to most likely have arisen from individuals guarding their food so that none of it is stolen by anyone at the table.
In the upper classes, the practice of being overly aggressive and placing your elbows in the table was seen as a sign of a common upbringing and so most nobles and gentry would look down on individuals who placed their elbows on the table.