In poker and some other card games: to ask to see the cards in another player's hand. This ends the bidding and the hand and reveals the winner.
This is closely related to the more general phrase: to call a bluff.
After the bidding went three rounds of raises, I called -- and lost my shirt.
He said he'd strip and streak through the championship football game for a hundred bucks. I didn't think he'd really do it so I put my money on the table and called his bluff. He lost his shirt and the rest of his clothes. And I lost $100.
A small penis.
This description is more common in the UK.
The guys teased Jack about his teenie weenie, but the girls still like having him around because he made up for the agate in his pants with other skills. What he lacked in natural dickdom he made up for by super-natural lickdom.