Primary definition: A whiny dramatic response by someone who is jealous of another person. The name Marsha could be changed with the name of any person you're mad at.
Secondary: Could be used as sarcastic reply or to make light of a seemingly ridiculous situation.
Gina: "Marsha Marsha Marsha!"
Amber: "My sister never shares the phone with me!"
Heather: "Marsha Marsha Marsha!"