put on也有“装出，假装“的意思，put on a brave face从字面上看意为”装出一副勇敢的面孔”, 引申意为“假装若无其事”。

You try to put on a brave face. 你装出一副若无其事的样子。

例如：
Tom : I have never experienced a roller coaster. I want you to sit beside me.
汤姆：我从来没做过过山车。你陪我坐好不好。
Bill: Wait a moment, I want to go to the toilet.
比利：等等，我先去个厕所。
Tom: You try to put on a brave face.
汤姆：你都是装的呀。