mutter about (someone or something)
1. To speak or utter something about someone or something in a very low, indistinct tone. Oh, don't mind me, I'm just muttering to myself about what's the fastest way to get home from here. I could hear the kids muttering something about a plan to cut school that day, but I couldn't make out the details of it.
2. To murmur, whisper, or grumble in an angry, disgruntled manner about someone or something. We all left the meeting muttering about the additional hours we'd have to work, but, in the end, what choice did we have? Instead of just muttering about the boss like you always do, why don't you register a complaint with HR?
mutter something about someone or something
to say something softly and indistinctly about someone or something. I heard him mutter something about being late. Sharon is muttering something about Dave. What does she mean?
mutter about someone or something
to grumble or complain about someone or something. Are you muttering about me? What is your complaint? Why is everyone muttering about the food here? It is excellent.