The staff at the store were quite unhelpful and didn't know where to find the product.

unhelpful 是由前缀 un-（表示"不、非"）和形容词 helpful（有帮助的）组成的复合词。helpful 来自动词 help（帮助）加上形容词后缀 -ful（表示"充满...的"）。unhelpful 字面意思是"不提供帮助的"，即"不愿帮忙的"。