I should have known better but I had to tell it to someone.

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"。 known better 是一个固定搭配，字面意思是"知道得更好"，引申为"明智地知道不该做某事"，常用来表达"本应知道不该那样做"的后悔或自责。