1. 深思熟虑;仔细的考虑
    If you do something without a second thought, you do it without thinking about it carefully, usually because you do not have enough time or you do not care very much.

    e.g. This murderous lunatic could kill them both without a second thought...
           这个杀人狂会不假思索地把他们两个都杀掉。
    e.g. Roberto didn''t give a second thought to borrowing $2,000 from him.
           罗伯托没有细想就从他那儿借了 2,000 美元。

2. 怀疑;疑虑
    If you have second thoughts about a decision that you have made, you begin to doubt whether it was the best thing to do.

second thought的翻译

    e.g. I had never had second thoughts about my decision to leave the company.
           我从未怀疑过自己当初离开公司的决定。

3. 转念间，经过重新考虑后(突然改变主意)
    You can say on second thoughts or on second thought when you suddenly change your mind about something that you are saying or something that you have decided to do.

    e.g. ''Wait there!'' Kathryn rose. ''No, on second thought, follow me.''
           “在那儿等着！”凯瑟琳站起身来，“不，我改主意了。跟我来。”

1. thinking again about a choice previously made

    e.g. he had second thoughts about his purchase

    Synonym: reconsiderationafterthoughtrethink