The compound adjective 'waist-deep' is a transparent formation from two native English elements. 'Waist', a Middle English term for the narrow part of the torso, combines with 'deep', an Old English word describing profound downward extent. The compound logically creates a measurement standard based on the human body, indicating a level of immersion or penetration up to the midpoint of the torso. This reflects a practical, descriptive method of quantifying depth in relation to a familiar anatomical landmark, evolving from literal physical description to a potential metaphor for being heavily involved or submerged in a situation.

Structure: waist (body part) + deep (measurement of depth) - waist: From Middle English 'wast', of uncertain origin, possibly from Old English wæst (growth, size). Refers to the part of the human body between the ribs and hips. - deep: From Old English 'dēop' (having great extent downward), from Proto-Germanic deupaz. Denotes a significant vertical dimension or penetration.