Who uses it: Baseball players
What it means: The infield fly rule exists to keep infielders from deliberately dropping fly balls in order to force double plays when runners are on first and second. Once you know that, the rule is simple. If the batting team has fewer than two outs and runners at first and second, and the batter hits a fly ball to the infield, the umpire can call the batter out before anyone catches the ball. The assumption is that a fielder would catch the ball, and the batting team benefits by getting only one out instead of a double play.
How you can use it: To describe something that seems arcane.
Today is the Portland Sea Dogs' home opener, but I'm not going, because 1) it's Holy Thursday, and there's Mass tonight and 2) I have too much work to do. It rained most of the morning, but the sun is out now, so the evening in Portland might even be pleasant.
My computer problems, which I'd hoped I could postpone dealing with for a while, have now reached a crisis point. Sigh. Excuse me while I go back everything up...