The word "willowy" is a straightforward English formation, combining the noun "willow" with the adjectival suffix "-y." The willow tree, with its distinctive long, slender, and pliant branches, provides the core visual metaphor. The suffix "-y," a common Germanic derivational tool, attaches to nouns to create adjectives meaning "full of" or "having the quality of." Therefore, the literal construction "willow-like" evolved to describe anything possessing the willow's characteristic grace and slenderness, most commonly applied to a person's physique. This semantic shift from a botanical feature to a human attribute exemplifies a classic metaphorical extension based on shared physical properties.

Structure: willow (tree type) + y (adjective-forming suffix) willow: From Old English welig, referring to the willow tree, known for its slender, flexible branches. y: A productive English suffix of Germanic origin, used to form adjectives meaning "characterized by" or "resembling."