1. 坦白的;直率的;坦诚的
    If someone is frank, they state or express things in an open and honest way.

frank的翻译

    e.g. 'It is clear that my client has been less than frank with me,' said his lawyer...
           “很明显我的委托人对我不够坦白，”他的律师说。
    e.g. They had a frank discussion about the issue.
           他们对这个问题进行了开诚布公的讨论。

2. (在信封或邮包上)盖邮资已付戳记，盖免费邮戳
    When a letter or parcel is franked, it is marked with a symbol that shows that the proper charge has been paid or that no stamp is needed.

frank

    e.g. The letter was franked in London on August 6.
           这封信 8 月 6 日在伦敦盖过免费邮戳。
    e.g. ...a self-addressed, franked envelope.
           署明回邮地址且已付邮资的信封

3. (尤用于观点可能令人不悦时)老实讲，坦白说
    You can say 'to be frank' or 'to be frank with you' to introduce a statement which is your honest opinion, especially when the person you are talking to might not like it.

    e.g. To be frank, he could also be a bit of a bore...
           老实说，他有时也会让人厌烦。
    e.g. To be frank with you, Harvey, I may have made a mistake.
           老实跟你讲，哈维，我可能犯了个错。