deep-blue

The compound adjective 'deep-blue' is a straightforward combination of two native English elements. 'Deep', with its ancient Germanic roots meaning profound or extending far down, evolved to describe intensity or darkness of a quality. 'Blue', entering English via French from a Germanic source, names the specific hue. When combined, the literal sense of 'a blue that is deep' creates a vivid color term, leveraging 'deep's metaphorical extension from physical depth to descriptive intensity, resulting in the modern meaning of a dark and saturated shade of blue.

Structure: deep (intense) + blue (color) - deep: From Old English dēop (deep, profound, serious), from Proto-Germanic deupaz. Functions as an intensifier describing degree or saturation. - blue: From Old French bleu, of Germanic origin (cf. Old High German blāo). Functions as the core noun denoting the color.