What this suffix does
-lipped forms compound adjectives that use the lips — the physical gateway of speech — to describe how much a person reveals, how they express emotion, and how tightly they control what they say. Tight-lipped, thin-lipped, loose-lipped, stiff-lipped — each compound uses the posture or character of the lips as a signal of the person's internal state and communicative intentions. These compounds are among the most visually precise in English: they make facial expression into character analysis. The lips are the hinge between the inner world and the spoken word.