1. Expression： sly dog
Explanation：Someone who, through cunning, devious, or dishonest means, achieves a fortuitous outcome. （偷鸡摸狗者，阴险狡猾的人）
Example：——We should keep Frank at arm's length. ——Yeah. He's known as a sly dog. He's done many people in.
Susan is a sly dog and we can' t depend on her.
He was beginning to believe that he had a sly dog to deal with indeed.
What a sly dog! He managed to get a copy of the test so he could memorize the answers ahead of time!
2. Expression：yellow dog
Explanation：A despicable person or thing. （卑鄙的人或事）
Example：I can't believe you get so invested in these debates—all politicians are yellow dogs that can't be trusted.
3. Expression：a big dog
Explanation：An important person. （重要的人）
Example：On arriving at the party, he saw a big dog. Then he went up and offered his hand, but he was proud and did not take it.
Come off your perch and stop considering yourself as the most biggest dog here.
He really seemed to look upon himself as biggest dog in the parish.