mark someone's card
mark (one's) card
To give someone pertinent information about something or someone else; to tip someone off. Shortly after I started, one of my co-workers marked my card about the boss's tendency to give very harsh criticism.
mark someone's cardgive someone information. informal
This idiom, which dates from the mid 20th century, derives from the world of horse racing. The card is a race card , the list of runners at a race meeting, so to mark someone's card is to give them tips for possible winners.