Meaning of “second thought” - Learner’s Dictionary

second thought

noun us uk
on second thoughts UK

used when you want to change a decision you have made:

I'll have tea, please - on second thoughts, make that coffee.
without a second thought

If you do something without a second thought, you do it without first considering if you should do it or not:

She'll spend a hundred pounds on a dress without a second thought.
have second thoughts

to change your opinion about something or start to doubt it:

[ + about + doing sth ] I've been having second thoughts about doing the course.

(Definition of “second thought” from the Cambridge Learner’s Dictionary © Cambridge University Press)