She has a weakness for designer bags, even though she knows it's bad for her budget.

weakness 是名词，来自形容词 weak（弱的），加上后缀 -ness 表示某种状态或性质。weakness 本义是"虚弱、弱点"，在这里引申为"对某事物的特别喜爱或无法抗拒的偏好"。