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.
-or-
Jane: Hey darling, do we need more vegetables?
Dave: Hrmm mm huh roomm.
Jane: What?
When one attempts to receive the cardboard drink holder from the drive-through window containing four large top-heavy beverages and looses grip, thus resulting in spillage of one or more beverages into one or more passengers' lap.
Berda attempted to receive the tray from the window attendant and spilled all over cody's lap, thus resulting in a McFumble.