Then, she prepared herself a stiff drink: gin and tonic with some ice, the very same one she used to have when Jack was there.

stiff 在这里是形容词，意思是"强烈的、浓烈的"，用来形容酒精含量高的酒。 drink 在这里特指"酒精饮料"，而不是普通的饮品。 stiff drink 是一个固定搭配，指"烈性酒"，通常指酒精浓度较高的调酒或纯饮。