give the game away

English edit

(idiomatic) To mistakenly reveal a secret.
You don't want to mention the money involved; that would give the game away.
- 2017, David Walliams [pseudonym; David Edward Williams], Bad Dad, London: HarperCollins Children’s Books, →ISBN:
  Just as the fearsome prison guard Mr Swivel was becoming more and more suspicious as to why these two had been under the table for so long, Flip slid up on to Dad’s chair. Without her glasses and wearing Dad’s prison overalls, she passed rather well for her nephew. Next, Dad slid up on to Flip’s chair. The boy had to stifle a giggle at the sight of his dad wearing one of Flip’s famous floaty dresses. The glasses softened his face, and from a distance he might just pass as the elderly librarian.
  “Stop giggling, mate!” hissed Dad. “You’ll give the game away.”

to mistakenly reveal a secret