withy n.

also weiə-, Proto-Indo-European root meaning "to turn, twist, bend," with derivatives referring to suppleness or binding. 

It forms all or part of: ferrule; garland; iridescence; iridescent; iris; iridium; vise; viticulture; wire; withe; withy.

It is the hypothetical source of/evidence for its existence is provided by: Avestan vaeiti- "osier;" Greek itea "willow," iris "rainbow;" Latin viere "to bend, twist," vitis "vine;" Lithuanian vytis "willow twig;" Old Irish fiar, Welsh gwyr "bent, crooked;" Polish witwa, Welsh gwden "willow," Russian vitvina "branch, bough;" Old English wir "metal drawn out into a fine thread." 

Old English welig "willow," from Proto-Germanic *wel- (source also of Old Saxon wilgia, Middle Dutch wilghe, Dutch wilg), probably from PIE root *wel- (3) "to turn, revolve," with derivatives referring to curved, enclosing objects. The change in form to -ow (14c.) paralleled that of bellow and fellow. The more typical Germanic word for the tree is represented by withy.