装嫩 means "to act cute" or "to act delicate" in Chinese. In English, it can be translated as "to act cute" or "to act delicate". I'm not sure of the exact origin of the phrase, but it likely comes from a desire to appear attractive or appealing. Here are some example sentences in English and their Chinese translations:

She always acts so cute around her crush. (她在她的暗恋对象面前总是装得很嫩。)

He pretends to be delicate, but he's actually quite tough. (他假装很娇气，但实际上他很坚强。)

She acts cute to get what she wants. (她装嫩来讨好别人。)