Which word means lacking intelligence or expression, often used to describe a person's demeanor?

The term "vacuous" accurately conveys the idea of lacking intelligence or expression, particularly when referring to a person's demeanor. It describes a state of emptiness or a lack of thought, which can manifest as a blank or unengaged expression. When someone is described as vacuous, it often implies a superficial or absent-minded quality, highlighting an absence of depth in their thoughts or emotions.

In contrast, "innocuous" refers to something that is harmless or not likely to provoke a strong reaction. "Obtuse" indicates a lack of sharpness or quickness in understanding, but does not necessarily imply a complete absence of intelligence. "Disingenuous" suggests insincerity or deceitfulness, focusing more on someone's honesty than their cognitive abilities or expressiveness. Therefore, in the context of describing a demeanor, "vacuous" is the most fitting choice, as it directly aligns with the attributes of lacking expression or intelligence.