1. Number one 最重要的人或事物)最好的人或事物)
Many people consider themselves number one, the most important person.
2. Look out for/ Take care of number one 先为自己着想，照顾自己的利益, 为自己打算
One and only 绝无仅有的特别的有名的
They are always looking out for number one and taking care of number one. It is as if they are the one and only person on Earth.
3. Pull a fast one 欺骗, 捣鬼
Jim only won the game because he pulled a fast one on his opponent.
4. One in a million百万分之一，凤毛麟角，百里挑一，佼佼者
She's one in a million.
5. At one time 同时, 曾经
At one time we met frequently.
6. Roll up into one 合为一体兼于一身
She is a great person -- kind, thoughtful and intelligent -- all good qualities rolled up into one.
7. One good turn deserves another. 善须善报要以德报德好人应得好报。
If he does you a favor, you should do him a favor. One good turn deserves another.
8. Be/go back to square one 退回起点, 从头再来
His papers were lost in the accident, so he had to go back to square one with the work.