This rule goes back to medieval times, when diners eat shoulder-to-shoulder at narrow feasting tables with no elbow room to spare. Imagine a big old medieval feast given by a ruler or lord. Everybody and their mother wants to go to it, and boast about how they dined with such and such. Most of these feasts would have been served on long tables with benches instead of seats. People would pack in like sardines and there simply was no room to have your elbows on the table without jostling your neighbor and disrupting his ability to eat. Allowing your elbows to nudge the guy next to you and hit his arm while he’s bringing some food to his mouth would be obviously rude. As well, such accidents could cause quarrels, or even violence to break out.
Picture Credit : Google