On second thought means having reconsidered. On second thought, let us leave it until Monday.
It marks a change of mind mid-conversation, which is what makes it a courtesy rather than a reversal. The British usually put an s on the end and say on second thoughts.
1phraseon second thoughtHaving reconsidered.
“On second thought I will take the train after all.”
on second thought is a phrase.
Also accepted in games: on second thoughts
Practised in Small Talk & Everyday Phrases