The compound adjective 'well-muscled' is a modern English formation built from native Germanic and borrowed Latinate elements. Its first component, 'well,' is an ancient Germanic adverb of manner, tracing back to Proto-Germanic wel and ultimately to the Proto-Indo-European root wel-, meaning "to wish, will," which evolved to signify doing something properly or satisfactorily. The core noun 'muscle' entered English via Old French from Latin musculus, a diminutive of mus ("mouse"), as the Romans likened the shape and movement of certain muscles, like the biceps, to a little mouse under the skin. The suffix '-ed,' of Old English origin, is added to nouns to form adjectives meaning "possessing" the noun's quality. Thus, the literal logic of the word is "characterized by having muscles in a good (developed, strong) manner."

Structure: well (in a good manner) + muscle (body tissue) + ed (having the quality of) - well: From Old English wel, meaning "in a good manner or way." It functions as an adverb modifying the adjective-forming compound. - muscle: From Latin musculus ("little mouse, muscle"), via French. It functions as the nominal root denoting the body tissue. - ed: From Old English -ede, an adjectival suffix meaning "having, provided with, characterized by."