He promised to keep the secret, but eventually ratted his friend out.

### 解释 ratted 是动词 rat 的过去式，rat 本意是"老鼠"，作为动词时引申为"告密、出卖"，常用于非正式场合。rat out 是固定搭配，表示"出卖某人"或"告发某人"。