The compound adjective "thigh-deep" is a straightforward descriptive formation from two native English elements. "Thigh" originates from the Old English þēoh, a Germanic word for the upper leg, while "deep" comes from the Old English dēop, meaning having great extension downward. The compound logically combines a specific body part with a measurement term to create a vivid, physical standard for depth, a common pattern in English for describing immersion (e.g., knee-deep, waist-deep). This construction reflects a practical, bodily frame of reference for quantifying one's experience within a substance like water or mud, evolving from literal descriptions to a standard metaphorical phrase for being heavily involved or immersed in a situation.

Structure: thigh (body part) + deep (measurement of depth) - thigh: From Old English þēoh, of Germanic origin, related to Dutch dij and German Dich. It functions as a noun specifying the body part used as a reference point for measurement. - deep: From Old English dēop, of Germanic origin, related to Dutch diep and German tief. It functions as an adjective describing the extent or measurement of depth.