Similar to "spoken for," when referring to relationship status, but more indistinct - when you are "mumbled for," you will hopefully be in a relationship soon, and it would be rude to engage in alternative romantic entanglements.
You might also be in a relationship with a person who mumbles.
Rob: Hey, you want to go enjoy the sunset together?
Jane: Sorry, I'm mumbled for - Dave and I have been seriously flirting for weeks now.
Jane: Hey darling, do we need more vegetables?
Dave: Hrmm mm huh roomm.