tip one's hand

verb2

1. •
   In card playing, to accidentally reveal one's cards or hand.
2. •
   (figurative) To inadvertently reveal any secret, particularly a secret that puts one at an advantage or disadvantage.

   • In Montreal, a 23-year-old pyromaniac tipped his hand by rushing into the fire station a few minutes before the gong rang, shouting: "Where's the fire?" — 1952