youthfulness comes from youthful + -ness, from youth, from Old English ġeoguþ ('youth'), from Proto-Germanic *jugunþiz, related to young. Morphologically, the word is transparent: -ness is attached to the older base named in the history, so the modern form packages an inherited or borrowed stem inside a later English derivative. In modern use, the noun sense is "the quality of being young or having the freshness of youth", which keeps the etymology connected to the word's present dictionary value rather than treating it as a merely historical label. The wider word family includes related English forms such as youthful, youth, young, showing how the same base continues to support neighboring English vocabulary. The principal historical forms in the chain include Old English ġeoguþ, Proto-Germanic *jugunþiz, which anchor the explanation in named linguistic material.