that made me hard just thinking of it.

在这个定语从句中，使役动词 made 接复合宾语：me 为宾语，形容词 hard 为宾补。hard 指人心情“难受”。

just thinking of it 是现在分词，作时间状语。等于说：when/as soon as I just thought of it（我一想到它时）。现在分词不可能做 made 的宾补。

to harbor ：暗藏着

promised a scent：显示出某种味道

I could have denied that his billowy blue shirt promised to harbor a scent of skin and sweat that made me hard just thinking of it.

【翻译】我本可以否认，他那波浪般的蓝色衬衫里有一种皮肤和汗水的味道，让我一想到它就觉得难受。