A Mouth Like A Sewer....I Think

Over the last two days I have noticed something new when Quinn and I are having a disagreement. When I tell her No or that she can't do something - she has started to mutter under her breath at me. She also does it to Greyson when he takes a toy or won't give her something she wants. She gives you this death stare and says "Pur Pur" very softly. When I ask what she is saying she says "I didn't say anything Mommy." I turn around and she will do it again. She only says it when she is mad and only when your back is turned. I am pretty much convinced that she has invented her own curse word and is basically cussing me out. Admittedly, I am at a loss as to what to do about it. Since I don't know the real meaning for sure I don't feel like I can punish her for it so I am just trying to ignore it. Am I the only one that has a two year old inventing curse words?? Someday I will appreciate my strong willed daughter, right?? She so inherited this from her father.