Surely two grown-ups should have known better than to shout at each other at the top of their lungs.

known 是动词 know 的过去分词形式，know 的意思是"知道、了解"。 better 是形容词 good 的比较级形式，意思是"更好的、更明智的"。 known better 是一个固定搭配，字面意思是"知道更好的做法"，实际表示"本应更明智/懂事"，暗指当事人本可以做出更好的行为选择。