Slang for text messaging.
The term thumb mumbling is often used for situations when someone is not paying attention to what is around them and decides to text people instead. For example: when driving, in class, hiking, at the beach, in a museum, on a date, during a meal, during a movie, a funeral, flying a plane or space ship, giving a speech, preforming surgery, etc.
Hey, are you thumb mumbling again? Put the f*cking phone down!