I did not research this, but remember using the term when playing pool. I always thought it was a play on the word Angle: when you use English, the targeted ball goes off at an angle.
I believe this has come up before, but I can’t find it.
In England, it’s called “side”.
The best information (really!) is that it was first introduced to American players by an Englishman, coincidentally named “English”.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
The official rule book, by the Billiards Congress of America, says the English figured out how to do it first, and tought American blokes how to do it when they held tournaments ‘n’ stuff there. So, for lack of imagination, the term ‘english’ stuck. I prefer to call it ‘the damn trick that doesn’t work for me and I end up pocketing the cue ball because of it’ maneuver.