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. This murderous lunatic could kill them both without a second thought Roberto didn't give a second thought to borrowing $2,000 from him
If you have second thoughts about a decision that you have made, you begin to doubt whether it was the best thing to do. I had never had second thoughts about my decision to leave the company
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. `Wait there!' Kathryn rose. `No, on second thought, follow me.'. A reconsideration of a decision or opinion previously made