in-depth

The compound adjective "in-depth" is a relatively modern formation, emerging in the early 20th century. It combines the spatial/figurative preposition "in," implying penetration or immersion, with the noun "depth," which itself evolved from the Old English adjective "dēop" (deep) via the abstract noun-forming suffix "-th." The logic of the word is transparently metaphorical: to be "in depth" is to go deeply into a subject, moving beyond the surface. Thus, an "in-depth" analysis is one that is thorough and comprehensive, conceptually exploring the profound or detailed aspects of a topic.

Structure: in (in) + depth (depth) - in: Old English, from Proto-Germanic, a preposition/adverb prefix meaning "in, into, on". - depth: Middle English, from Old English dēop (deep) + the noun-forming suffix -th (as in length, strength), meaning "the quality or state of being deep".