Give the game away
Definition 1 of 1
Pro Tip 1/3Secret Spoiling
Used when someone accidentally or carelessly reveals a secret, ruining a surprise. 
Be careful what you say, you might give the game away about her surprise birthday party.
Pro Tip 2/3Informal Context
This phrase is casual, often used among friends and family more than in formal settings. 
Oops, I think I just gave the game away about our road trip to Mark.
Pro Tip 3/3Figurative Use
The phrase is usually not about an actual game; it's a metaphor for disclosing information. 
She gave the game away about the upcoming merger by leaving the confidential report on her desk.