the adverb "thoroughly" is built directly upon its adjective form "thorough." the adjective "thorough" itself originates from the old english word thurh, meaning "through." the semantic evolution is clear: the concept of going all the way through a task or subject naturally extended to mean doing it with completeness and attention to every part. the adverbial suffix "-ly," of germanic origin, is one of the most productive in english, routinely converting adjectives into adverbs. thus, "thoroughly" literally means "in a through or complete manner," perfectly capturing its modern sense of exhaustive action or absolute degree.

Structure: thorough + ly - thorough: from old english thurh (through), with functional role as the base adjective meaning complete. - ly: from old english -līce, an adverbial suffix forming adverbs from adjectives.