His adorable chubbiness made him the favorite of the whole family.

chubbiness 是由形容词 chubby（圆胖的、丰满的）加上名词后缀 -ness 构成的名词，表示"圆胖的状态或特征"。chubby 本身来自名词 chub（一种圆胖的鱼），引申为形容人胖乎乎的样子。