wikipedia

Some believe that clock faces were used to keep score on court, with a quarter move of the minute hand to indicate a score of 15, 30, and 45. When the hand moved to 60, the game was over. However, in order to ensure that the game could not be won by a one-point difference in players' scores, the idea of "deuce" was introduced. To make the score stay within the 60 ticks on the clock face, the 45 was changed to 40. Therefore, if both players had 40, the first player to score would receive ten, and that would move the clock to 50. If that player scored a second time in a row, they would be awarded another ten and the clock would move to 60, signifying the end of the game. However, if the player failed to score twice in a row, then the clock would move back to 40 to establish another "deuce".