A tongo player was a player of the card game tongo.
Jadzia Dax was an avid tongo player.
In 2371, Julian Bashir accused Quark of allowing Jadzia to win at tongo because he was hoping to gain her affections. Jadzia denied the accusation. When Deral interrupted their argument, Bashir explained he had been telling Jadzia that she wasn't as good a tongo player as she believed she was. Jadzia retorted that she was very good at the game. (DS9: "Meridian")
In 2374, Miles O'Brien claimed Worf knew nothing about the game, which the Klingon admitted. He then confessed a newfound appreciation for tongo after having married a tongo player. (DS9: "Change of Heart")