Downy means covered in very soft fine hair. A downy cheek, downy feathers, downy and warm.
Down is the layer of fluffy feathers under the outer ones, and it insulates by trapping air rather than by being thick — which is why a down jacket is light and warm and useless when wet. Of a face it means the first soft hair of youth.
1adjectivedownyCovered in very soft fine hair.
“The newly hatched ducklings were soft and downy.”
downy is a quality of softness.
Middle English
doun
English
down
English, today
downy
Built from
From down + -y. down comes from Middle English doun. From Middle English doun, doune (“down”), from Old English dūne (“down”), aphetic form of adūne (“down, downward”), from earlier ofdūne (“down”) literally “off the hill”, from of (“of, off of”) + dūn (“hill, mount, dune, down”). More at Etymology 2 below. For the development from directional phrases to prepositions, compare dene, dāle, whence dal. Compare also stq, del. Cognate with Scots doon (“down”).
Etymology from Wiktionary, CC BY-SA