In betting parlance, a lock is an easy winner, or a clear no lose situation. It may refer to the bettor themselves, or the team or competitors being wagered on. In may also be used to refer to the losing side, such as, "the New England Patriots are a lock to lose the division."
It is common superstition to never say a particular bet, game, or outcome is a lock before the game goes final. It is bad luck to do so. There are times at the end of regular season games, known as garbage time, when both teams put their second and third string players on the field because the chance of winning or losing the lead are very slim. In this case, it would be safe for an individual who bet on the game to say they are a lock to win (assuming they bet on the right team).