show one's hand

To make one's plans, intentions, ideas, or resources known to others, especially those that were previously hidden or kept secret. (Also expressed as "show (one's) cards"; both phrases refer to displaying one's cards ("hand") during a card game.) In business negotiations, it's important that you don't show your hand right away, or you might risk losing out on the best deal possible. Pressure from the government is forcing the notoriously secretive CEO to show his hand regarding his company's tax profile and offshore accounts.

Reveal one's plans, intentions, or resources, especially when they were previously hidden. For example, We have to be careful not to show our hand to our competitors. The hand here refers to a hand of cards, and showing them means turning them face up. [Late 1800s]

To reveal one’s true motives or intentions. This term, like laying one’s cards on the table, comes from card-playing. Edmund Campion used a version of it in 1581: “I would I might be suffered to shewe my cardes” (Conferences Held in the Tower of London with Ed. Campion, Jesuit, cited by OED). A variant is to tip one’s hand, of the same provenance. The Economist used it on November 17, 1979: “Mr Hunt will not tip his hand on the price at which he will buy more bullion.”