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饮品。 drink 是名词，指"饮料、酒水"，在这里特指含酒精的饮品。 stiff drink 是一个固定搭配，指"烈性酒、酒精浓度高的饮品"，通常指威士忌、金酒等烈酒调配的鸡尾酒。