Here are some of the possible reasons why someone might talk under their breath:
* They are thinking out loud: They might be working through a problem or brainstorming ideas and expressing their thoughts without realizing they are speaking.
* They are nervous or shy: They might feel uncomfortable speaking loudly in public or around others.
* They are angry or upset: They might be expressing their frustration or anger in a low voice, possibly to avoid confrontation.
* They are secretive: They might be whispering something they don't want others to hear.
The meaning of "talking under your breath" can depend on the context. It's important to consider the situation and the speaker's body language to get a better understanding of their intentions.