slack-jawed

The compound adjective "slack-jawed" is a vivid, literal descriptor from the late 19th century, combining the Old English concept of laxness ("slack") with the physical feature ("jawed"). It directly paints the image of a jaw hanging loosely open due to muscular relaxation. This literal depiction of a dropped jaw evolved naturally into a metaphor for a state of stunned surprise, bewilderment, or simple-mindedness, where one's mental faculties are as inactive and "slack" as the physical jaw. The logic is transparent: the physical posture of an open, limp jaw visually signifies a lapse in attentive control, whether from shock or a lack of intelligence.

Structure: slack (loose, drooping) + jawed (having a jaw) slack: From Old English slæc (loose, lax, careless), of Germanic origin. Functions as an adjective describing a lack of tension. jawed: From Middle English jawe, of uncertain origin. The suffix -ed forms an adjective meaning "having or provided with."