How would you define a state of being quiet or not clearly expressed?

The term that best describes a state of being quiet or not clearly expressed is "tacit." Tacit refers to something that is understood or implied without being directly stated or openly expressed. It captures the essence of silence in communication, indicating an understanding that exists without the need for overt articulation.

In contexts where information or emotions are communicated non-verbally or subtly, "tacit" conveys that there may be an agreement, acknowledgment, or understanding, but it is not articulated in a direct manner. This idea of something being unspoken aligns perfectly with the meaning you are searching for in this question.

The other terms do not fit this definition: terse implies brevity in speech but is often direct; stymie refers to blocking or preventing progress; and sullen describes a mood of sulkiness or gloominess. None of these capture the idea of being quiet or unexpressed in the same way as "tacit" does.