The word "willow" traces its lineage back to the Old English welig, a term for the tree. This Old English form is cognate with the Old Saxon wilgia and the Middle Low German wilge, all pointing to a common West Germanic ancestor. The ultimate origin is likely a Pre-Germanic root, possibly related to the Proto-Indo-European root wel- meaning "to turn, twist," a fitting source for a tree whose pliant branches are famously used for weaving and basketry. Over centuries, the phonetic form evolved from welig through Middle English wilwe to the modern "willow," while its meaning has remained consistently tied to the specific, flexible tree.

The modern English word "willow" is a simplex form derived from the Old English welig, which is not productively segmentable into smaller meaningful parts within modern English.