In almost all cases, it only means he has a big mouth. For the other meaning, check the definitions of "nhiều chuyện" (southern dialect, used verbally), "ngồi lê đôi mách" (northern dialect, verbal and literature).
I would add "bà tám"..... ><
Does that only mean that his mouth is large, or does it also mean, as in English, that he can't keep secrets?
Is he a clown at the circus? <sub>.</sub>
Look up Joe E. Brown. A comedian with the biggest mouth I have ever seen.