Learner's definition of SECOND THOUGHT
a feeling of guilt, doubt, worry, etc., that you have after you have decided to do something or after something has happened
After she agreed to lend him the money, she had second thoughts.
She was having second thoughts about getting married.
They throw away perfectly good food without (so much as) a second thought. [=without worrying about it]
Don't give a second thought to [=don't worry about] that broken vase.
He hardly gave it a second thought.
on second thought
on second thoughts
after thinking about something again