In the game of baseball, the pitcher throws the ball over a piece of rubber called home plate. A man with a long round piece of wood tries to hit the ball if it is over the piece of rubber. If he hits the ball, he gets to run to a base. If he swings and misses, a man behind home plate called an umpire calls "strike!" If the ball is between certain distances above home plate and the batter does not swing, the umpire calls, "strike." If he gets three strikes he is "out." He lost his chance to hit the ball and run. He struck out. That is a strikeout.
The term carries over to other areas of life. A man calls a girl for a date. He calls her three times. Three times she turns him down. "Did you get a date with her?" "No, I struck out." At that point he gives up. A salesman tries to sell his product. The third time he strikes out, or he gives up. (Some do not give up). Three strikes, You are out, comes from the game of baseball. It is used in a whole lot more areas of life.