Originated in about 1983. From a Knock knock joke. About the time (your mamma) jokes started. It was in place of YO short for YOUR. Hence JO
Knock Knock...Who's there? Jo.. Jo Who? JOMAMMA.
Basically it means "your mom" when the word Jomamma is said alone the reader must use his immagination as to what it means. it could mean your mom is fat or your mom is ugly.
Player 1: Your so gay
Player 2: Jomamma's gay <_<