second thought

WordReference Random House Learner's Dictionary of American English © 2025

ˈsec•ond ˈthought, n. [countable]

1. Often, second thoughts. [plural] an act of reconsidering a previous action, position, etc., often because of doubts as to its correctness:
   We had second thoughts about firing her.

WordReference Random House Unabridged Dictionary of American English © 2025

sec′ond thought′, 

1. Often, second thoughts. reservation about a previous action, position, decision, judgment, or the like:
   He had second thoughts about his decision.
2. on second thought, after reconsideration:
   On second thought, I don't think I'll go.
Also, sec′ond-thought′. 
• 1625–35

Collins Concise English Dictionary © HarperCollins Publishers::

second thought noun

1. (usually plural) a revised opinion or idea on a matter already considered