eat my left is a phrased used at a person who deserves utter disrespect.
when saying this at someone you are telling them to get down and worship your left nut as they are useless people. touching your right is totally unacceptable.
when using the comeback "eat my left", no matter what they say you have won the argument
Person 1: Geez you've got a head like a kicked in coke can!
Person 2: eat my left ya piece of shit