Definition
Needless or unnecessary speech.
How People Actually Use It
When someone is useless to a degree that the air they breathe is considered to be going to waste It’s an exaggerated insult meant as sarcasm for someone or group of people screwing something up which is extremely simple and in million years you don’t expect anyone to screw it so using this phrase you are questioning them consuming the air in the atmosphere
"He was asked to bring a cake to the party and he showed up with a 5 cupcakes, it is such a waste of human breath She literally burnt eggs while boiling them, she is a waste of human breath They were to take a family picture where they seemed normal and after 30 attempts they couldn’t it, they are just waste of human breath"
Source: Urban Dictionary