What do we mean when we say it today?
After all this time, the phrase still means something is unfair, underhanded, or devious. Most of the time we come across this phrase today, it has something to do with sports. People say there was "foul play" in the game, or even shout "foul play" at the ref. Makes sense because sports have fouls, right?
Another place we hear this phrase is on crime shows on TV (or in real life crime for that matter.) All Law and Order junkies know what we're saying: "Police suspect foul play in the murder of the congressman."
But sometimes we still hear it out in the world without sports or crime to guide it. And then it still just means anything that's unfair. People can toss out this term in any context. It's even a Chevy Chase movie.