trump

Козырь means “trump” in the sense of a decisive advantage, especially a high card in card games or a strong argument in a negotiation. It is commonly used in phrases such as главный козырь (“main trump card”) and держать козырь в рукаве (“to have a trump card up one’s sleeve”).

It is a masculine noun; its plural is козыри. In card-game contexts, козырь can also mean the trump suit.

1. Этот аргумент — его главный козырь.
   This argument is his main trump card.
2. У нашей команды есть козырь в рукаве.
   Our team has a trump card up its sleeve.