the word 'youthfully' is a modern english formation built upon the old english noun 'geoguth', which itself evolved from proto-germanic 'jugunþ-', denoting the state of being young. the adjectival suffix '-ful' was attached to create 'youthful', meaning 'full of the qualities of youth'. finally, the adverbial suffix '-ly', with its origins in old english '-lice' meaning 'body' or 'form', was added to describe the manner of an action. thus, the literal construction 'in a manner full of youth' logically evolved to describe actions performed with the vitality, energy, or appearance characteristic of young people.

Structure: youth (young) + ful (full of) + ly (in a manner of) youth: from old english 'geoguth', meaning the state or quality of being young. ful: from old english '-ful', an adjectival suffix meaning 'full of' or 'characterized by'. ly: from old english '-lice', an adjectival suffix that forms adverbs meaning 'in a manner of'.