Why was "tantrum" trending last week?
Old English welig, from Proto-Germanic *walg- (cf. Old Saxon wilgia, Middle Dutch wilghe, Dutch wilg), probably from PIE *wel- "to turn, roll," 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.