From which language is the tennis term "deuce" derived?

The scoring system of tennis is arcane and the origins are not well understood. It is likely tennis derives from a game played in medieval France in which a clock face was used to keep score. Points in the games were incremented in multiples of fifteen (the “forty” call is thought simply to be short for “forty-five,” and sixty, the top score, was never called as the game ended when this score was reached.

When a game is at the 40-40 mark and a player still needs to win by two clear points, then it goes to deuce. This is where a player must first score to gain advantage in the game, then score the next point to win. The etymology of the term “deuce” is a corruption of the French phrase ‘a deux de jeu’ ‘two points’ away from the (end of the) game. “Deuce” means that the two players are equal in score and that each player would need to win two consecutive points to win the game.

Even though the term “deuce” has its roots in French, it is not used in the French Open. The French prefer to use the term “égalité” (“equality”) at Stade Roland Garros in Paris.