Show (one's) hand idiom
If you show your hand, you disclose or uncover your aim, purpose, opinion, etc. to someone else, especially those that you used to hide from view or conceal from those people.
Laura is not the type of person who tends to show her hand. She keeps secrets very well.
Wait until tonight to let your mother know about your plan of moving abroad. Don't show your hand too early, especially when she is not in a good mood like now.
Showing your hand immediately when you have not known a person well is not a wise thing to do.
To let a secret out by accident
To talk freely and unintentionally disclose private information or secret to others
To accidentally reveal what you are going to do or what you believe
1. To stop talking or thinking about something.
2. To unintentionally disclose important or confidential information.
The idiom dated back to late 1800s. The hand here refers to a hand of cards, and showing them means turning them face up.
If you fall for someone's trick the first time then they are at fault.
Are you going to mess with me again? Fool me once, shame on you; fool me twice, shame on me.